The FAA landing field length requirements for jet airplanes are specified in 14 CFR part 25. It defines the minimum field length (and therefore minimum margins) that can be scheduled. The regulation describes the landing profile as the horizontal distance required to land and come to a complete stop on a dry surface runway from a point 50 feet above the runway threshold, through the flare and touchdown, using the maximum stopping capability of the aircraft. The unfactored or certified landing distance is determined during aircraft certification. As such, it may be different from the actual landing distance because certification regulations do not take into account all factors that could potentially affect landing distance. The unfactored landing distance is the baseline landing distance on a dry, level runway at standard temperatures without using thrust reversers, auto brakes, or auto-land systems. In order to meet regulatory requirements however, a safety margin of 67 percent is added to the unfactored dry landing distance in the FAA-approved AFM, after applicable adjustments are made for environmental and aircraft conditions (MEL/CDL penalties). This corrected length is then referred to as the factored dry-landing distance or the minimum dry-landing field length. [Figure 15-22]
For minimum wet-landing field length, the factored drylanding distance is increased by an additional 15 percent. Thus, the minimum dry runway field length is 1.67 times the actual minimum air and ground distance needed, and the wet runway minimum landing field length is 1.92 times the minimum dry air and ground distance needed.
Certified landing field length requirements are computed for the stop made with speed brakes deployed and maximum wheel braking. Reverse thrust is not used in establishing the certified landing distances; however, reversers should definitely be used in service.
As in the takeoff planning, there are certain speeds that must be taken into consideration when landing a jet airplane. The speeds are as follows:
- VSO—stall speed in the landing configuration
- VREF—1.3 times the stall speed in the landing configuration
- Approach climb—the speed that guarantees adequate performance in a go-around situation with an inoperative engine. The airplane’s weight must be limited so that a twin-engine airplane has a 2.1 percent climb gradient capability. (The approach climb gradient requirements for 3 and 4 engine airplanes are 2.4 percent and 2.7 percent, respectively.) These criteria are based on an airplane configured with approach flaps, landing gear up, and takeoff thrust available from the operative engine(s).
- Landing climb—the speed that guarantees adequate performance in arresting the descent and making a go-around from the final stages of landing with the airplane in the full landing configuration and maximum takeoff power available on all engines.
The appropriate speeds should be pre-computed prior to every landing and posted where they are visible to both pilots. The VREF speed, or threshold speed, is used as a reference speed throughout the traffic pattern. For example:
- Downwind leg—VREF plus 20 knots
- Base leg—VREF plus 10 knots
- Final approach—VREF plus 5 knots
- 50 feet over threshold—VREF
The approach and landing sequence in a jet airplane should be accomplished in accordance with an approach and landing profile developed for the particular airplane. [Figure 15-23]
A safe approach in any type of airplane culminates in a particular position, speed, and height over the runway threshold. That final flight condition is the target window at which the entire approach aims. Propeller-powered airplanes are able to approach that target from wider angles, greater speed differentials, and a larger variety of glidepath angles. Jet airplanes are not as responsive to power and course corrections, so the final approach must be more stable, more deliberate, and more constant in order to reach the window accurately.
The transitioning pilot must understand that, in spite of their impressive performance capabilities, there are six ways in which a jet airplane is worse than a piston-engine airplane in making an approach and in correcting errors on the approach.
- The absence of the propeller slipstream in producing immediate extra lift at constant airspeed. There is no such thing as salvaging a misjudged glidepath with a sudden burst of immediately available power. Added lift can only be achieved by accelerating the airframe. Not only must the pilot wait for added power but, even when the engines do respond, added lift is only available when the airframe has responded with speed.
- The absence of the propeller slipstream in significantly lowering the power-on stall speed. There is virtually no difference between power-on and power-off stall speed. It is not possible in a jet airplane to jam the thrust levers forward to avoid a stall.
- Poor acceleration response in a jet engine from low rpm. This characteristic requires that the approach be flown in a high drag/high power configuration so that sufficient power is available quickly if needed.
- The increased momentum of the jet airplane making sudden changes in the flightpath impossible. Jet airplanes are consistently heavier than comparable sized propeller airplanes. The jet airplane, therefore, requires more indicated airspeed during the final approach due to a wing design that is optimized for higher speeds. These two factors combine to produce higher momentum for the jet airplane. Since force is required to overcome momentum for speed changes or course corrections, the jet is far less responsive than the propeller airplane and requires careful planning and stable conditions throughout the approach.
- The lack of good speed stability being an inducement to a low-speed condition. The drag curve for many jet airplanes is much flatter than for propeller airplanes, so speed changes do not produce nearly as much drag change. Further, jet thrust remains nearly constant with small speed changes. The result is far less speed stability. When the speed does increase or decrease, there is little tendency for the jet airplane to re-acquire the original speed. The pilot, therefore, must remain alert to the necessity of making speed adjustments, and then make them aggressively in order to remain on speed.
- Drag increasing faster than lift producing a high sink rate at low speeds. Jet airplane wings typically have a large increase in drag in the approach configuration. When a sink rate does develop, the only immediate remedy is to increase pitch attitude (AOA). Because drag increases faster than lift, that pitch change rapidly contributes to an even greater sink rate unless a significant amount of power is aggressively applied.
These flying characteristics of jet airplanes make a stabilized approach an absolute necessity.