Basic Aircraft Flight

To the pilot transitioning into jet airplanes, going faster is seldom a problem. It is getting the airplane to slow down that seems to cause the most difficulty. This is because of the extremely clean aerodynamic design and fast momentum of the jet airplane and because the jet lacks the propeller drag effects that the pilot has been accustomed to. Additionally, even with the power reduced to flight idle, the jet engine still produces thrust, and deceleration of the jet airplane is a slow process. Jet airplanes have a glide performance that is double that of piston-powered airplanes, and jet pilots often cannot comply with an ATC request to go down and slow down at the same time. Therefore, jet airplanes are equipped with drag devices, such as spoilers and speed brakes.

The primary purpose of spoilers is to spoil lift. The most common type of spoiler consists of one or more rectangular plates that lie flush with the upper surface of each wing. They are installed approximately parallel to the lateral axis of the airplane and are hinged along the leading edges. When deployed, spoilers deflect up against the relative wind, which interferes with the flow of air about the wing. [Figure 15-18] This both spoils lift and increases drag. Spoilers are usually installed forward of the flaps but not in front of the ailerons so as not to interfere with roll control.

Deploying spoilers results in a substantial sink rate with little decay in airspeed. Some airplanes exhibit a nose-up pitch tendency when the spoilers are deployed, which the pilot must anticipate.

When spoilers are deployed on landing, most of the wing’s lift is destroyed. This action transfers the airplane’s weight to the landing gear so that the wheel brakes are more effective. Another beneficial effect of deploying spoilers on landing is that they create considerable drag, adding to the overall aerodynamic braking. The real value of spoilers on landing, however, is creating the best circumstances for using wheel brakes.

The primary purpose of speed brakes is to produce drag. Speed brakes are found in many sizes, shapes, and locations on different airplanes, but they all have the same purpose—to assist in rapid deceleration. The speed brake consists of a hydraulically-operated board that, when deployed, extends into the airstream. Deploying speed brakes results in a rapid decrease in airspeed. Typically, speed brakes can be deployed at any time during flight in order to help control airspeed, but they are most often used only when a rapid deceleration must be accomplished to slow down to landing gear and flap speeds. There is usually a certain amount of noise and buffeting associated with the use of speed brakes, along with an obvious penalty in fuel consumption. Procedures for the use of spoilers and/or speed brakes in various situations are contained in the FAA-approved AFM for the particular airplane.

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The stalling characteristics of the sweptwing jet airplane can vary considerably from those of the normal straight wing airplane. The greatest difference that will be noticeable to the pilot is the lift developed vs. angle of attack. An increase in angle of attack of the straight wing produces a substantial and constantly increasing lift vector up to its maximum coefficient of lift, and soon thereafter flow separation (stall) occurs with a rapid deterioration of lift.

By contrast, the sweptwing produces a much more gradual buildup of lift with a less well-defined maximum coefficient. This less-defined peak also means that a swept wing may not have as dramatic loss of lift at angles of attack beyond its maximum lift coefficient. However, these high-lift conditions are accompanied by high drag, which results in a high rate of descent. [Figure 15-13]

The differences in the stall characteristics between a conventional straight wing/low tailplane (non T-tail) airplane and a sweptwing T-tail airplane center around two main areas.

• The basic pitching tendency of the airplane at the stall.
• Tail effectiveness in stall recovery.

On a conventional straight wing/low tailplane airplane, the weight of the airplane acts downwards forward of the lift acting upwards, producing a need for a balancing force acting downwards from the tailplane. As speed is reduced by gentle up elevator deflection, the static stability of the airplane causes a nose-down tendency. This is countered by further up elevator to keep the nose coming up and the speed decreasing. As the pitch attitude increases, the low set tail is immersed in the wing wake, which is slightly turbulent, low energy air. The accompanying aerodynamic buffeting serves as a warning of impending stall. The reduced effectiveness of the tail prevents the pilot from forcing the airplane into a deeper stall. [Figure 15-14] The conventional straight wing airplane conforms to the familiar nose-down pitching tendency at the stall and gives the entire airplane a fairly pronounced nosedown pitch. At the moment of stall, the wing wake passes more or less straight rearward and passes above the tail. The tail is now immersed in high energy air where it experiences a sharp increase in positive AOA causing upward lift. This lift then assists the nose-down pitch and decrease in wing AOA essential to stall recovery.

In a sweptwing jet with a T-tail and rear fuselage mounted engines, the two qualities that are different from its straight wing low tailplane counterpart are the pitching tendency of the airplane as the stall develops and the loss of tail effectiveness at the stall. The handling qualities down to the stall are much the same as the straight wing airplane except that the high, T-tail remains clear of the wing wake and provides little or no warning in the form of a pre-stall buffet. Also, the tail is fully effective during the speed reduction towards the stall, and remains effective even after the wing has begun to stall. This enables the pilot to drive the wing into a deeper stall at a much greater AOA.

At the stall, two distinct things happen. After the stall, the sweptwing T-tail airplane tends to pitch up rather than down, and the T-tail can become immersed in the wing wake, which is low energy turbulent air. This greatly reduces tail effectiveness and the airplane’s ability to counter the noseup pitch. Also, if the AOA increases further, the disturbed, relatively slow air behind the wing may sweep across the tail at such a large angle that the tail itself stalls. If this occurs, the pilot loses all pitch control and will be unable to lower the nose. The pitch up just after the stall is worsened by large reduction in lift and a large increase in drag, which causes a rapidly increasing descent path, thus compounding the rate of increase of the wing’s AOA. [Figure 15-15]

A slight pitch up tendency after the stall is a characteristic of a swept or tapered wings. With these types of wings, there is a tendency for the wing to develop a spanwise airflow towards the wingtip when the wing is at high angles of attack. This leads to a tendency for separation of airflow, and the subsequent stall, to occur at the wingtips first. [Figure 15-16]

As previously stated, when flying at a speed near VMD, an increase in AOA causes drag to increase faster than lift and the airplane begins to sink. It is essential to understand that this increasing sinking tendency, at a constant pitch attitude, results in a rapid increase in AOA as the flightpath becomes deflected downwards. [Figure 15-17] Furthermore, once the stall has developed and a large amount of lift has been lost, the airplane will begin to sink rapidly and this will be accompanied by a corresponding rapid increase in AOA. This is the beginning of what is termed a deep stall.

As an airplane enters a deep stall, increasing drag reduces forward speed to well below normal stall speed. The sink rate may increase to many thousands of feet per minute. It must be emphasized that this situation can occur without an excessively nose-high pitch attitude. On some airplanes, it can occur at an apparently normal pitch attitude, and it is this quality that can mislead the pilot because it appears similar to the beginning of a normal stall recovery. It can also occur at a negative pitch attitude, that is, with the nose pointing towards the ground. In such situations, it seems counterintuitive to apply the correct recovery action, which is to push forward on the pitch control to reduce the AOA, as this action will also cause the nose to point even further towards the ground. But, that is the right thing to do.

Deep stalls may be unrecoverable. Fortunately, they are easily avoided as long as published limitations are observed. On those airplanes susceptible to deep stalls (not all swept or tapered wing airplanes are), sophisticated stall warning systems such as stick shakers are standard equipment. A stick pusher, as its name implies, acts to automatically reduce the airplane’s AOA before the airplane reaches a dangerous stall condition, or it may aid in recovering the airplane from a stall if an airplane’s natural aerodynamic characteristics do so weakly.

Pilots undergoing training in jet airplanes are taught to recover at the first sign of an impending stall instead of going beyond those initial cues and into a full stall. Normally, this is indicated by aural stall warning devices or activation of the airplane’s stick shaker. Stick shakers normally activate around 107 percent of the actual stall speed. In response to a stall warning, the proper action is for the pilot to apply a nose-down input until the stall warning stops (pitch trim may be necessary). Then, the wings are rolled level, followed by adjusting thrust to return to normal flight. The elapsed time will be small between these actions, particularly at low altitude where a significant available thrust exists. It is important to understand that reducing AOA eliminates the stall, but applying thrust will allow the descent to be stopped once the wing is flying again.

At high altitudes the stall recovery technique is the same. A pilot will need to reduce the AOA by lowering the nose until the stall warning stops. However, after the AOA has been reduced to where the wing is again developing efficient lift, the airplane will still likely need to accelerate to a desired airspeed. At high altitudes where the available thrust is significantly less than at lower altitudes, the only way to achieve that acceleration is to pitch the nose downwards and use gravity. As such, several thousand feet or more of altitude loss may be needed to recover completely. The above discussion covers most airplanes; however, the stall recovery procedures for a particular make and model airplane may differ slightly, as recommended by the manufacturer, and are contained in the FAA-approved Airplane Flight Manual for that airplane.

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The jet airplane wing, designed primarily for high speed flight, has relatively poor low speed characteristics. As opposed to the normal piston powered airplane, the jet wing has less area relative to the airplane’s weight, a lower aspect ratio (long chord/short span), and thin airfoil shape—all of which amount to the need for speed to generate enough lift. The sweptwing is additionally penalized at low speeds because its effective lift is proportional to airflow speed that is perpendicular to the leading edge. This airflow speed is always less than the airspeed of the airplane itself. In other words, the airflow on the sweptwing has the effect of persuading the wing into believing that it is flying slower than it actually is.

The first real consequence of poor lift at low speeds is a high stall speed. The second consequence of poor lift at low speeds is the manner in which lift and drag vary at those low speeds. As a jet airplane is slowed toward its minimum drag speed (V_MD or L/D_MAX), total drag increases at a much greater rate than the changes in lift, resulting in a sinking flightpath. If the pilot attempts to increase lift by increasing the AOA, airspeed will be further reduced resulting in a further increase in drag and sink rate as the airplane slides up the back side of the power-required curve. The sink rate can be arrested in one of two ways:

• Pitch attitude can be substantially reduced to reduce the AOA and allow the airplane to accelerate to a speed above V_MD, where steady flight conditions can be reestablished. This procedure, however, will invariably result in a substantial loss of altitude.
• Thrust can be increased to accelerate the airplane to a speed above V_MD to reestablish steady flight conditions. The amount of thrust must be sufficient to accelerate the airplane and regain altitude lost. Also, if the airplane has slid a long way up the back side of the power required (drag) curve, drag will be very high and a very large amount of thrust will be required.

In a typical piston engine airplane, V_MD in the clean configuration is normally at a speed of about 1.3 V_S. [Figure 15-12] Flight below V_MD on a piston engine airplane is well identified and predictable. In contrast, in a jet airplane flight in the area of V_MD (typically 1.5 – 1.6 V_S) does not normally produce any noticeable changes in flying qualities other than a lack of speed stability—a condition where a decrease in speed leads to an increase in drag which leads to a further decrease in speed and hence a speed divergence. A pilot who is not cognizant of a developing speed divergence may find a serious sink rate developing at a constant power setting, and a pitch attitude that appears to be normal. The fact that drag increases more rapidly than lift, causing a sinking flightpath, is one of the most important aspects of jet airplane flying qualities.

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Thus far, only the Mach buffet that results from excessive speed has been addressed. The transitioning pilot, however, should be aware that Mach buffet is a function of the speed of the airflow over the wing— not necessarily the airspeed of the airplane. Anytime that too great a lift demand is made on the wing, whether from too fast an airspeed or from too high an angle of attack (AOA) near the M_MO, the “high speed buffet” will occur. However, there are also occasions when the buffet can be experienced at much slower speeds known as “low speed Mach buffet.”

The most likely situations that could cause the low speed buffet would be when an airplane is flown at too slow of a speed for its weight and altitude causing a high AOA. This very high AOA would have the same effect of increasing airflow over the upper surface of the wing to the point that all of the same effects of the shock waves and buffet would occur as in the high speed buffet situation.

The AOA of the wing has the greatest effect on inducing the Mach buffet, or pre-stall buffet, at either the high or low speed boundaries for the airplane. The conditions that increase the AOA, hence the speed of the airflow over the wing and chances of Mach buffet are:

• High altitudes—The higher the airplane flies, the thinner the air and the greater the AOA required to produce the lift needed to maintain level flight.
• Heavy weights—The heavier the airplane, the greater the lift required of the wing, and all other things being equal, the greater the AOA.
• “G” loading—An increase in the “G” loading of the wing results in the same situation as increasing the weight of the airplane. It makes no difference whether the increase in “G” forces is caused by a turn, rough control usage, or turbulence. The effect of increasing the wing’s AOA is the same.

An airplane’s indicated airspeed decreases in relation to true airspeed as altitude increases. As the indicated airspeed decreases with altitude, it progressively merges with the low speed buffet boundary where pre-stall buffet occurs for the airplane at a load factor of 1.0 G. The point where the high speed Mach indicated airspeed and low speed buffet boundary indicated airspeed merge is the airplane’s absolute or aerodynamic ceiling. This is where if an airplane flew any slower it would exceed its stalling AOA and experience low speed buffet. Additionally, if it flew any faster it would exceed M_MO, potentially leading to high speed buffet. This critical area of the airplane’s flight envelope is known as “coffin corner.” All airplanes are equipped with some form of stall warning system. Crews must be aware of systems installed on their airplanes (stick pushers, stick shakers, audio alarms, etc.) and their intended function. In a high altitude environment, airplane buffet is sometimes the initial indicator of problems.

Mach buffet occurs as a result of supersonic airflow on the wing. Stall buffet occurs at angles of attack that produce airflow disturbances (burbling) over the upper surface of the wing which decreases lift. As density altitude increases, the AOA that is required to produce an airflow disturbance over the top of the wing is reduced until the density altitude is reached where Mach buffet and stall buffet converge (coffin corner). When this phenomenon is encountered, serious consequences may result causing loss of airplane control.

Increasing either gross weight or load factor (G factor) will increase the low speed buffet and decrease Mach buffet speeds. A typical jet airplane flying at 51,000 feet altitude at 1.0 G may encounter Mach buffet slightly above the airplane’s M_MO (0.82 Mach) and low speed buffet at 0.60 Mach. However, only 1.4 G (an increase of only 0.4 G) may bring on buffet at the optimum speed of 0.73 Mach and any change in airspeed, bank angle, or gust loading may reduce this straight-and-level flight 1.4 G protection to no protection at all. Consequently, a maximum cruising flight altitude must be selected which will allow sufficient buffet margin for necessary maneuvering and for gust conditions likely to be encountered. Therefore, it is important for pilots to be familiar with the use of charts showing cruise maneuver and buffet limits. [Figure 15-11]

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The typical piston-powered airplane had to deal with two maximum operating speeds:

• V_NO—maximum structural cruising speed, represented on the airspeed indicator by the upper limit of the green arc. It is, however, permissible to exceed V_NO and operate in the caution range (yellow arc) in certain flight conditions.
• V_NE—never-exceed speed, represented by a red line on the airspeed indicator.

These speed margins in the piston airplanes were never of much concern during normal operations because the high drag factors and relatively low cruise power settings kept speeds well below these maximum limits.

Maximum speeds in jet airplanes are expressed differently and always define the maximum operating speed of the airplane, which is comparable to the V_NE of the piston airplane. These maximum speeds in a jet airplane are referred to as:

• V_MO—maximum operating speed expressed in terms of knots.
• M_MO—maximum operating speed expressed in terms of a decimal of Mach speed (speed of sound).

To observe both limits V_MO and M_MO, the pilot of a jet airplane needs both an airspeed indicator and a Machmeter, each with appropriate red lines. In some general aviation jet airplanes, these are combined into a single instrument that contains a pair of concentric indicators: one for the indicated airspeed and the other for indicated Mach number. Each is provided with an appropriate red line. [Figure 15-8]

Because of the higher available thrust and very low drag design, the jet airplane can very easily exceed its speed margin even in cruising flight and, in fact, in some airplanes in a shallow climb. The handling qualities in a jet can change drastically when the maximum operating speeds are exceeded.

High-speed airplanes designed for subsonic flight are limited to some Mach number below the speed of sound to avoid the formation of shock waves that begin to develop as the airplane nears Mach 1.0. These shock waves (and the adverse effects associated with them) can occur when the airplane speed is substantially below Mach 1.0. The Mach speed at which some portion of the airflow over the wing first equals Mach 1.0 is termed the critical Mach number (Mcr). This is also the speed at which a shock wave first appears on the airplane.

There is no particular problem associated with the acceleration of the airflow up to Mach Crit, the point where Mach 1.0 is encountered; however, a shock wave is formed at the point where the airflow suddenly returns to subsonic flow. This shock wave becomes more severe and moves aft on the wing as speed of the wing is increased and eventually flow separation occurs behind the well-developed shock wave. [Figure 15-9]

If allowed to progress well beyond the M_MO for the airplane, this separation of air behind the shock wave can result in severe buffeting and possible loss of control or “upset.” Because of the changing center of lift of the wing resulting from the movement of the shock wave, the pilot experiences pitch change tendencies as the airplane moves through the transonic speeds up to and exceeding M_MO. [Figure 15-10]

As the graph in Figure 15-10 illustrates, initially as speed is increased up to Mach .72, the wing develops an increasing amount of lift requiring a nose-down force or trim to maintain level flight. With increased speed and the aft movement of the shock wave, the wing’s center of pressure also moves aft causing the start of a nose-down tendency or “tuck.” By Mach .9, the nose-down forces are well developed to a point where a total of 70 pounds of back pressure are required to hold the nose up. If allowed to progress unchecked, Mach tuck may eventually occur. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane could enter a steep, sometimes unrecoverable, dive.

An alert pilot would have observed the high airspeed indications, experienced the onset of buffeting, and responded to aural warning devices long before encountering the extreme stick forces shown. However, in the event that corrective action is not taken and the nose is allowed to drop, increasing airspeed even further, the situation could rapidly become dangerous. As the Mach speed increases beyond the airplane’s M_MO, the effects of flow separation and turbulence behind the shock wave become more severe. Eventually, the most powerful forces causing Mach tuck are a result of the buffeting and lack of effective downwash on the horizontal stabilizer because of the disturbed airflow over the wing. This is the primary reason for the development of the T-tail configuration on some jet airplanes, which places the horizontal stabilizer as far as practical from the turbulence of the wings. Also, because of the critical aspects of high-altitude/high-Mach flight, most jet airplanes capable of operating in the Mach speed ranges are designed with some form of trim and autopilot Mach compensating device (stick puller) to alert the pilot to inadvertent excursions beyond its certificated M_MO.

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Absence of Propeller Effect

The absence of a propeller has a significant effect on the operation of jet-powered airplanes that the transitioning pilot must become accustomed to. The effect is due to the absence of lift from the propeller slipstream and the absence of propeller drag.

Absence of Propeller Slipstream

A propeller produces thrust by accelerating a large mass of air rearwards, and (especially with wing-mounted engines) this air passes over a comparatively large percentage of the wing area. On a propeller-driven airplane, the lift that the wing develops is the sum of the lift generated by the wing area not in the wake of the propeller (as a result of airplane speed) and the lift generated by the wing area influenced by the propeller slipstream. By increasing or decreasing the speed of the slipstream air, it is possible to increase or decrease the total lift on the wing without changing airspeed.

For example, a propeller-driven airplane that is allowed to become too low and too slow on an approach is very responsive to a quick blast of power to salvage the situation. In addition to increasing lift at a constant airspeed, stalling speed is reduced with power on. A jet engine, on the other hand, also produces thrust by accelerating a mass of air rearward, but this air does not pass over the wings. Therefore, there is no lift bonus at increased power at constant airspeed and no significant lowering of power-on stall speed.

In not having propellers, the jet-powered airplane is minus two assets:

• It is not possible to produce increased lift instantly by simply increasing power.
• It is not possible to lower stall speed by simply increasing power. The 10-knot margin (roughly the difference between power-off and power-on stall speed on a propeller-driven airplane for a given configuration) is lost.

Add the poor acceleration response of the jet engine, and it becomes apparent that there are three ways in which the jet pilot is worse off than the propeller pilot. For these reasons, there is a marked difference between the approach qualities of a piston-engine airplane and a jet. In a piston-engine airplane, there is some room for error. Speed is not too critical and a burst of power salvages an increasing sink rate. In a jet, however, there is little room for error.

If an increasing sink rate develops in a jet, the pilot must remember two points in the proper sequence:

1. Increased lift can be gained only by accelerating airflow over the wings, and this can be accomplished only by accelerating the entire airplane.
2. The airplane can be accelerated, assuming altitude loss cannot be afforded, only by a rapid increase in thrust, and here, the slow acceleration of the jet engine (possibly up to 8 seconds) becomes a factor.

Salvaging an increasing sink rate on an approach in a jet can be a very difficult maneuver. The lack of ability to produce instant lift in the jet, along with the slow acceleration of the engine, necessitates a “stabilized approach” to a landing where full landing configuration, constant airspeed, controlled rate of descent, and relatively high power settings are maintained until over the threshold of the runway. This allows for almost immediate response from the engine in making minor changes in the approach speed or rate of descent and makes it possible to initiate an immediate go-around or missed approach if necessary.

Absence of Propeller Drag

When the throttles are closed on a piston-powered airplane, the propellers create a vast amount of drag, and airspeed is immediately decreased or altitude lost. The effect of reducing power to idle on the jet engine, however, produces no such drag effect. In fact, at an idle power setting, the jet engine still produces forward thrust. The main advantage is that the jet pilot is no longer faced with a potential drag penalty of a runaway propeller or a reversed propeller. A disadvantage, however, is the “freewheeling” effect forward thrust at idle has on the jet. While this occasionally can be used to advantage (such as in a long descent), it is a handicap when it is necessary to lose speed quickly, such as when entering a terminal area or when in a landing flare. The lack of propeller drag, along with the aerodynamically clean airframe of the jet, are new to most pilots, and slowing the airplane down is one of the initial problems encountered by pilots transitioning into jets.

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Thrust To Thrust Lever Relationship

In a piston-engine, propeller-driven airplane, thrust is proportional to rpm, manifold pressure, and propeller blade angle, with manifold pressure being the most dominant factor. At a constant rpm, thrust is proportional to throttle lever position. In a jet engine, however, thrust is quite disproportional to thrust lever position. This is an important difference that the pilot transitioning into jet-powered airplanes must become accustomed to.

On a jet engine, thrust is proportional to rpm (mass flow) and temperature (fuel/air ratio). These are matched and a further variation of thrust results from the compressor efficiency at varying rpm. The jet engine is most efficient at high rpm, where the engine is designed to be operated most of the time. As rpm increases, mass flow, temperature, and efficiency also increase. Therefore, much more thrust is produced per increment of throttle movement near the top of the range than near the bottom.

One thing that seems different to the piston pilot transitioning into jet-powered airplanes is the rather large amount of thrust lever movement between the flight idle position and full power as compared to the small amount of movement of the throttle in the piston engine. For instance, an inch of throttle movement on a piston may be worth 400 horsepower wherever the throttle may be. On a jet, an inch of thrust lever movement at a low rpm may be worth only 200 pounds of thrust, but at a high rpm that same inch of movement might amount to closer to 2,000 pounds of thrust. Because of this, in a situation where significantly more thrust is needed and the jet engine is at low rpm, it does not do much good to merely “inch the thrust lever forward.” Substantial thrust lever movement is in order. This is not to say that rough or abrupt thrust lever action is standard operating procedure. If the power setting is already high, it may take only a small amount of movement. However, there are two characteristics of the jet engine that work against the normal habits of the piston-engine pilot. One is the variation of thrust with rpm, and the other is the relatively slow acceleration of the jet engine.

Variation of Thrust with RPM

Whereas piston engines normally operate in the range of 40 percent to 70 percent of available rpm, jets operate most efficiently in the 85 percent to 100 percent range, with a flight idle rpm of 50 percent to 60 percent. The range from 90 percent to 100 percent in jets may produce as much thrust as the total available at 70 percent. [Figure 15-6]

Slow Acceleration of the Jet Engine

In a propeller-driven airplane, the constant speed propeller keeps the engine turning at a constant rpm within the governing range, and power is changed by varying the manifold pressure. Acceleration of the piston from idle to full power is relatively rapid, somewhere on the order of 3 to 4 seconds. The acceleration on the different jet engines can vary considerably, but it is usually much slower.

Efficiency in a jet engine is highest at high rpm where the compressor is working closest to its optimum conditions. At low rpm, the operating cycle is generally inefficient. If the engine is operating at normal approach rpm and there is a sudden requirement for increased thrust, the jet engine responds immediately and full thrust can be achieved in about 2 seconds. However, at a low rpm, sudden fullpower application tends to over fuel the engine resulting in possible compressor surge, excessive turbine temperatures, compressor stall and/or flameout. To prevent this, various limiters, such as compressor bleed valves, are contained in the system and serve to restrict the engine until it is at an rpm at which it can respond to a rapid acceleration demand without distress. This critical rpm is most noticeable when the engine is at idle rpm, and the thrust lever is rapidly advanced to a high-power position. Engine acceleration is initially very slow, but can change to very fast after about 78 percent rpm is reached. [Figure 15-7]

Even though engine acceleration is nearly instantaneous after about 78 percent rpm, total time to accelerate from idle rpm to full power may take as much as 8 seconds. For this reason, most jets are operated at a relatively high rpm during the final approach to landing or at any other time that immediate power may be needed.

Jet Engine Efficiency

Maximum operating altitudes for general aviation turbojet airplanes now reach 51,000 feet. The efficiency of the jet engine at high-altitudes is the primary reason for operating in the high-altitude environment. The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant engine rpm and true airspeed (TAS). Thus, by flying at a high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the most advantageous cruise speed. For efficiency, jet airplanes are typically operated at high altitudes where cruise is usually very close to rpm or EGT limits. At high altitudes, little excess thrust may be available for maneuvering. Therefore, it is often impossible for the jet airplane to climb and turn simultaneously, and all maneuvering must be accomplished within the limits of available thrust and without sacrificing stability and controllability.

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In a jet engine, thrust is determined by the amount of fuel injected into the combustion chamber. The power controls on most turbojet-and turbofan-powered airplanes consist of just one thrust lever for each engine, because most engine control functions are automatic. The thrust lever is linked to a fuel control and/or electronic engine computer that meters fuel flow based upon revolutions per minute (rpm), internal temperatures, ambient conditions, and other factors. [Figure 15-3]

In a jet engine, each major rotating section usually has a separate gauge devoted to monitoring its speed of rotation. Depending on the make and model, a jet engine may have an N₁ gauge that monitors the low-pressure compressor section and/or fan speed in turbofan engines. The gas generator section may be monitored by an N₂ gauge, while triple spool engines may have an N₃ gauge as well. Each engine section rotates at many thousands of rpm. Their gauges therefore are calibrated in percent of rpm rather than actual rpm, for ease of display and interpretation. [Figure 15-4]

The temperature of turbine gases must be closely monitored by the pilot. As in any gas turbine engine, exceeding temperature limits, even for a very few seconds, may result in serious heat damage to turbine blades and other components. Depending on the make and model, gas temperatures can be measured at a number of different locations within the engine. The associated engine gauges therefore have different names according to their location. For instance:

• Exhaust Gas Temperature (EGT)—the temperature of the exhaust gases as they enter the tail pipe after passing through the turbine.
• Turbine Inlet Temperature (TIT)—the temperature of the gases from the combustion section of the engine as they enter the first stage of the turbine. The TIT is the highest temperature inside a gas turbine engine and is one of the limiting factors of the amount of power the engine can produce. TIT, however, is difficult to measure. Therefore, EGT, which relates to TIT, is normally the parameter measured.
• Interstage Turbine Temperature (ITT)—the temperature of the gases between the high-pressure and low-pressure turbine wheels.
• Turbine Outlet Temperature (TOT)—like EGT, turbine outlet temperature is taken aft of the turbine wheel(s).

Jet Engine Ignition

Most jet engine ignition systems consist of two igniter plugs, which are used during the ground or air starting of the engine. Once the start is completed, this ignition either automatically goes off or is turned off, and from this point on, the combustion in the engine is a continuous process.

Continuous Ignition

An engine is sensitive to the flow characteristics of the air that enters the intake of the engine nacelle. So long as the flow of air is substantially normal, the engine continues to run smoothly. However, particularly with rear-mounted engines that are sometimes in a position to be affected by disturbed airflow from the wings, there are some abnormal flight situations that could cause a compressor stall or flameout of the engine. These abnormal flight conditions would usually be associated with abrupt pitch changes such as might be encountered in severe turbulence or a stall.

In order to avoid the possibility of engine flameout from the above conditions, or from other conditions that might cause ingestion problems, such as heavy rain, ice, or possible bird strike, most jet engines are equipped with a continuous ignition system. This system can be turned on and used continuously whenever the need arises. In many jets, as an added precaution, this system is normally used during takeoffs and landings. Many jets are also equipped with an automatic ignition system that operates both igniters whenever the airplane stall warning or stick shaker is activated.

Fuel Heaters

Because of the high altitudes and extremely cold outside air temperatures in which the jet flies, it is possible to supercool the jet fuel to the point that the small particles of water suspended in the fuel can turn to ice crystals and clog the fuel filters leading to the engine. For this reason, jet engines are normally equipped with fuel heaters. The fuel heater may be of the automatic type that constantly maintains the fuel temperature above freezing, or they may be manually controlled by the pilot.

Setting Power

On some jet airplanes, thrust is indicated by an engine pressure ratio (EPR) gauge. EPR can be thought of as being equivalent to the manifold pressure on the piston engine. EPR is the difference between turbine discharge pressure and engine inlet pressure. It is an indication of what the engine has done with the raw air scooped in. For instance, an EPR setting of 2.24 means that the discharge pressure relative to the inlet pressure is 2.24:1. On these airplanes, the EPR gauge is the primary reference used to establish power settings. [Figure 15-5]

Fan speed (N₁) is the primary indication of thrust on most turbofan engines. Fuel flow provides a secondary thrust indication, and cross-checking for proper fuel flow can help in spotting a faulty N₁ gauge. Turbofans also have a gas generator turbine tachometer (N₂). They are used mainly for engine starting and some system functions.

In setting power, it is usually the primary power reference (EPR or N₁) that is most critical and is the gauge that first limits the forward movement of the thrust levers. However, there are occasions where the limits of either rpm or temperature can be exceeded. The rule is: movement of the thrust levers must be stopped and power set at whichever the limits of EPR, rpm, or temperature is reached first.

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A jet engine is a gas turbine engine. A jet engine develops thrust by accelerating a relatively small mass of air to very high velocity, as opposed to a propeller, which develops thrust by accelerating a much larger mass of air to a much slower velocity.

Piston and gas turbine engines are internal combustion engines and have a similar basic cycle of operation; that is, induction, compression, combustion, expansion, and exhaust. Air is taken in and compressed, and fuel is injected and burned. The hot gases then expand and supply a surplus of power over that required for compression and are finally exhausted. In both piston and jet engines, the efficiency of the cycle is improved by increasing the volume of air taken in and the compression ratio.

Part of the expansion of the burned gases takes place in the turbine section of the jet engine providing the necessary power to drive the compressor, while the remainder of the expansion takes place in the nozzle of the tail pipe in order to accelerate the gas to a high velocity jet thereby producing thrust. [Figure 15-1]

In theory, the jet engine is simpler and more directly converts thermal energy (the burning and expansion of gases) into mechanical energy (thrust). The piston or reciprocating engine, with all of its moving parts, must convert the thermal energy into mechanical energy and then finally into thrust by rotating a propeller.

One of the advantages of the jet engine over the piston engine is the jet engine’s capability of producing much greater amounts of thrust horsepower at the high altitudes and high speeds. In fact, turbojet engine efficiency increases with altitude and speed.

Although the propeller-driven airplane is not nearly as efficient as the jet, particularly at the higher altitudes and cruising speeds required in modern aviation, one of the few advantages the propeller-driven airplane has over the jet is that maximum thrust is available almost at the start of the takeoff roll. Initial thrust output of the jet engine on takeoff is relatively lower and does not reach peak efficiency until the higher speeds. The fanjet or turbofan engine was developed to help compensate for this problem and is, in effect, a compromise between the pure jet engine (turbojet) and the propeller engine.

Like other gas turbine engines, the heart of the turbofan engine is the gas generator—the part of the engine that produces the hot, high-velocity gases. Similar to turboprops, turbofans have a low-pressure turbine section that uses most of the energy produced by the gas generator. The low pressure turbine is mounted on a concentric shaft that passes through the hollow shaft of the gas generator, connecting it to a ducted fan at the front of the engine. [Figure 15-2]

The fan-jet concept increases the total thrust of the jet engine, particularly at the lower speeds and altitudes. Although efficiency at the higher altitudes is lost (turbofan engines are subject to a large lapse in thrust with increasing altitude), the turbofan engine increases acceleration, decreases the takeoff roll, improves initial climb performance, and often has the effect of decreasing specific fuel consumption. Specific fuel consumption is a ratio of the fuel used by an engine and the amount of thrust it produces.

Flight Literacy Recommends

Rod Machado's Private Pilot Handbook -Flight Literacy recommends Rod Machado's products because he takes what is normally dry and tedious and transforms it with his characteristic humor, helping to keep you engaged and to retain the information longer. (see all of Rod Machado's Products).