Effect of Nonstandard Pressure and Temperature
If no means were provided for adjusting altimeters to nonstandard pressure, flight could be hazardous. For example, if a flight is made from a high-pressure area to a low-pressure area without adjusting the altimeter, the actual altitude of the glider is lower than indicated altitude. When flying from a low-pressure area to a high-pressure area, the actual altitude of the glider is higher than the indicated altitude. Fortunately, this error can be corrected by setting the altimeter properly. When flying in the local area and communication with ATC is not available to provide the current altimeter setting, there is an alternative procedure. Set the altimeter on zero while on the ground to directly read AGL referenced to the landing zone. However, for cross-country flights and flights from higher elevation airports, that procedure may not be an option due to the excessive Kollsman window setting instrument required. In those instances, the altimeter can be set to read the field elevation and either disregard the Kollsman window setting or applying a correction factor, or set in the barometric altimeter setting and remember if it reads high or low.
Variations in air temperature also affect the altimeter. On a warm day, the expanded air is lighter in weight per unit volume than on a cold day, and consequently the pressure levels are raised. For example, the pressure level at which the altimeter indicates 10,000 feet is higher on a warm day than under standard conditions. On a cold day, the reverse is true, and the 10,000-foot level would be lower. The adjustment made by the pilot to compensate for nonstandard pressures does not compensate for nonstandard temperatures. Therefore, if terrain or obstacle clearance is a factor in the selection of a cruising altitude, particularly at higher altitudes, remember to anticipate that colder than standard temperature places the glider lower than the altimeter indicates. Therefore, a higher altitude should be used to provide adequate terrain clearance. A memory aid in applying the above is “from a high to a low or hot to cold, look out below.” [Figure 4-18]
Setting the Altimeter (Kollsman Window)
To adjust the altimeter for variation in atmospheric pressure, the pressure scale in the altimeter setting window (Kollsman Window), calibrated in inches of mercury (“Hg), is adjusted to correspond with the given altimeter setting. Altimeter settings can be defined as station pressure reduced to sea level, expressed in inches of mercury. Pilots should be aware of three altimeter setting acronyms: QNH, QFE, and QNE. The pilot sets QNH to read MSL, sets QFE to read the altitude above ground level (AGL), and QNE when flying above the transition level (18,000 feet in the United States). A glider pilot would use this setting when wave flying and is above 18,000 feet or flight level 180 (FL180). To read QNH, pilots should set the current altimeter setting into the altimeter’s barometric setting window. The altimeter should read what the approximate field elevation is, which is a good way to check if it is working properly. A good memory aid for this step is to think of “H” (QNH) for home. QFE is set at zero when on the field. As a memory aid, think of the F in QFE as “field.” Many altimeters cannot be set to read zero when operating from the higher elevation airports.
The station reporting the altimeter setting takes an hourly measurement of the station’s atmospheric pressure and corrects this value to sea level pressure. These altimeter settings reflect height above sea level only in the vicinity of the reporting station. Aircraft flying above the transition level (in the United States, FL180), must use the standard altimeter setting of 29.92 “Hg (QNE). When flying below 18,000 feet MSL, it is necessary to adjust the altimeter setting as the flight progresses from one station to the next.
When flying over high mountainous terrain, certain atmospheric conditions can cause the altimeter to indicate an altitude of 1,000 feet, or more, higher than the actual altitude. For this reason, a generous margin of altitude should be allowed—not only for possible altimeter error, but also for possible downdrafts that are particularly prevalent if high winds are encountered.
To illustrate the use of the altimeter setting system, follow a cross-country flight from TSA Gliderport, Midlothian, Texas, to Winston Airport, Snyder, Texas, via Stephens County Airport, Breckenridge, Texas. Before takeoff from TSA Gliderport, the pilot receives a current local altimeter setting of 29.85 from the Fort Worth automated flight service station (AFSS). This value is set in the altimeter setting window of the altimeter. The altimeter indication should then be compared with the known airport elevation of 660 feet. Since most altimeters are not perfectly calibrated, an error may exist. VFR flights altimeters do not need to be calibrated but if an altimeter indication differs from the field elevation by more than 75 feet, the accuracy of the instrument is questionable, and it should be referred to an instrument repair station.
When over Stephens County Airport, assume the pilot receives a current area altimeter setting of 29.94 and applies this setting to the altimeter. Before entering the traffic pattern at Winston Airport, a new altimeter setting of 29.69 is received from the Automated Weather Observing System (AWOS), and applied to the altimeter. If the pilot desires to enter the traffic pattern at approximately 1,000 feet above terrain, and the field elevation of Winston Airport is 2,430 feet, an indicated altitude of 3,400 feet should be used.
2,430 feet + 1,000 feet = 3,430 feet, rounded to 3,400 feet
The importance of properly setting and reading the altimeter cannot be overemphasized. Let us assume that the pilot neglected to adjust the altimeter at Winston Airport to the current setting, and uses the Stephens County area setting of 29.94. If this occurred, the glider would be approximately 250 feet below the airport downwind entry altitude of 3,200 feet when entering the Winston Airport traffic pattern, and the altimeter would indicate approximately 250 feet higher than the field elevation (2,430 feet) upon landing.
This altitude error is another reason for a glider pilot to always fly a safe visual approach angle to the landing zone and not solely depend on instrument readings or past selection of turn points for the traffic pattern. The visual angle to the landing zone is the one item of information that will always be true. Reports of wind and barometric pressure if available may still be suspect. The glider pilot must always ensure sufficient altitude is available for expected or possible winds to allow landing in the landing zone.
Actual altimeter setting = 29.94
Correct altimeter setting = 29.69
Difference = .25
One inch of pressure is equal to approximately 1,000 feet of altitude.
.25 × 1,000 feet = 250 feet
The previous calculation may be confusing, particularly in determining whether to add or subtract the amount of altimeter error. The following additional explanation is offered and can be helpful in finding the solution to this type of problem.
There are two means by which the altimeter pointers can be moved. One utilizes changes in air pressure, while the other utilizes the mechanical makeup of the altimeter setting system.
When the glider altitude is changed, the changing pressure within the altimeter case expands or contracts the aneroid barometer that, through linkage, rotates the pointers. A decrease in pressure causes the altimeter to indicate an increase in altitude, and an increase in pressure causes the altimeter to indicate a decrease in altitude. If the glider is flown from a pressure level of 28.75 “Hg to a pressure level of 29.75 “Hg, the altimeter would show a decrease of approximately 1,000 feet in altitude.
The other method of moving the pointers does not rely on changing air pressure, but on the mechanical construction of the altimeter. When the knob on the altimeter is rotated, the altimeter setting pressure scale moves simultaneously with the altimeter pointers. This may be confusing because the numerical values of pressure indicated in the window increase while the altimeter indicates an increase in altitude, and decrease while the altimeter indicates a decrease in altitude. This is contrary to the reaction on the pointers when air pressure changes and is based solely on the mechanical makeup of the altimeter. To further explain this point, assume that the correct altimeter setting is 29.50, or a .50 difference. This would cause a 500 foot error in altitude. In this case, if the altimeter setting is adjusted from 30.00 to 29.50, the numerical value decreases and the altimeter indicates a decrease of 500 feet in altitude. Before this correction was made, the glider was actually flying at an altitude of 500 feet lower than was shown on the altimeter. It is important to calibrate the altimeter for all VFR flights because if they are off by more than a comfortable margin, calibration or repair is advisable.