One of the benefits of a standard atmosphere lies in the concepts of pressure altitude (PA) and density altitude (DA). PA is that altitude in the standard atmosphere which corresponds to a particular static air pressure. An aircraft altimeter senses pressure through the static portion of the pitot-static system, then shows the altitude at which that pressure would be found in the standard atmosphere. The pitot-static system will be discussed in more detail in Chapter Two.
All early altimeters were referenced to the standard Sea Level pressure of 29.92 inches Hg at sea level. Altitude was estimated by determining the altitude in the standard atmosphere at which the measured pressure would occur. Modern altimeters can be adjusted to yield accurate altitude
at a known point, like an airfield, by changing reference sea level pressure to an appropriate value. Airport meteorologists measure air pressure, determine what pressure at sea level would have to exist to yield an accurate altitude reading using standard pressure lapse rate, and report that setting to pilots. After being set properly, an adjustable altimeter reports the altitude at which the measured pressure would be found if a standard pressure lapse rate was applied to a sea level pressure equal to that set by the pilot. Standard pressure lapse rate is 1000 feet of PA for each one inch of Hg. To determine PA (in the standard atmosphere) for use in calculations a pilot needs only to set 29.92 inches the altimeter and read the result.
Properly setting the altimeter, and periodically checking for changes in the prescribed setting can be very important. To properly set the altimeter, pilots should make sure that they are receiving the altimeter setting in inches of mercury for height above sea level. Three types of settings may be encountered overseas: QNH, QNE, and QFE.
1. QNH is altitude corrected to standard sea level (provided in ATIS in the United States).
2. QNE is PA (altimeter set to 29.92)
3. QFE is a setting at an airfield to read height above ground at that location (the altimeter would read zero at the airfield surface).
The issue overseas is further confused by the fact that settings are often given in hectopascals (millibars), rather than inches of mercury. To avoid problems, listen for foreign controllers’
statements of what their pressure reference is and, if necessary use the Flight Information Handbook to convert.
Checking for changes in altimeter setting during cross country travel or as weather moves in can also be important. When station pressure or temperature drops, an altimeter set at the previous condition will read higher than it should. For example, as depicted in figure 1-10 below, a flight at 1000 ft AGL in the standard atmosphere would have the aircraft at a pressure of 28.92. Note that flight into a high pressure system would cause the altimeter to read low (actual altitude of 2000 ft AGL) while flight into a low pressure system would cause the altimeter to read higher than actual altitude (0 ft AGL).
REMEMBER
“High to low look out below”, or "hot to cold look out below.”
Figure 1-10 Pressure Altitude vs. True Altitude with Varying Local Pressures 112. ALTITUDE COMPUTATIONS – DENSITY ALTITUDE
A more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude (DA). DA is that altitude in the standard atmosphere which corresponds to a particular air density. Changes in air density are caused by variations in
atmospheric pressure, temperature, and humidity. Density altitude (DA) is the PA corrected for temperature and humidity deviations from the standard atmosphere. If you know the outside air temperature and humidity, and you can determine the PA, as described above, you can derive the DA. DA is the environmental factor which most significantly affects power available.
Typically, aviators will use a chart (Figure 1-11) to determine DA for the ambient PA and temperature. Enter the chart at the bottom at the appropriate outside air temperature (OAT) and plot vertically upward to intersect the current PA depicted on the diagonals, determined by dialing 29.92 into the aircraft altimeter. From this point, read laterally to the left to determine the DA (not corrected for humidity). In the example depicted on Figure 1-11, a temperature of 6 °C and 2400’ PA, the DA is 2000’.
Another method for estimating DA is to use the ‘rule of thumb’ equation below.
DA = PA + [(TAmbient – TStd@Altitude)]X120 For PA, dial in 29.92 " Hg. Temps are in C.
The above ‘rule of thumb’ merely requires an accurate understanding of standard temperature at altitude. Remember that the temperature at sea level for a standard day is 15 °C. With an average lapse rate of 2 °C / 1000’ MSL, the standard temperature at altitude can be easily
determined; i.e., 5 °C at 5000’ MSL, and -5 °C at 10,000’ MSL. This estimation of DA still fails to correct for humidity, which is discussed below.
As mentioned earlier, changes in the water vapor content, or humidity, can also greatly affect the density of the air, in addition to temperature deviations from standard. To recap, as humidity increases, water molecules with less mass and approximately the same volume as air molecules displace the more dense air molecules to make the same overall volume contain less actual mass.
Thus, an increase in humidity leads to a decrease in air density, and, therefore, an increase in DA.
Figure 1-11 DA Chart
One way to adjust calculations for humidity is to use a higher temperature than might be
associated with lower density in performance charts. This fictional quantity is known as virtual temperature, and is defined as OAT corrected for relative humidity. In the same way that wind chill is applied to a cold day’s temperature to reflect how the wind affects the human body, a virtual temperature correction may be applied to a temperature measurement to reflect the effect of humidity on the air’s density. The genesis of this virtual temperature concept for military pilots was possibly an old 1962 Navy Weather Research Facility pamphlet that rigorously defined the equations involved in DA computations and illustrated their relationships for practical application using a DA Circular Computer. These computers are still in use today at many weather offices. The dew point temperature correction chart (Figure 1-12) serves the same purpose as a DA Computer. It accepts dew point and temperature, and then yields virtual
temperature and DA. Regardless of method used, the question you might want to ask your weather briefer is if virtual temperature (moisture effect) was used in the computation of the DA given on your weather brief.
Figure 1-12 Dew Point Correction Chart
Moisture in the air can be slightly beneficial in controlling engine temperature, but generally tends to be detrimental to helicopter performance. Moisture as liquid can provide valuable cooling to an engine. For example, water is sprayed into the inlet of an AV-8B to cool the airflow during high demand operations so that maximum power can be obtained without exceeding material heat limits on turbine blades.
As the water evaporates, it cools the airflow leading to the engine turbine section allowing higher combustion temperatures and an increase in thrust produced. But moisture as vapor in the air is also a performance degrader. The decrease in the density of the airflow due to the presence of water molecules requires more mass flow of air to produce the same amount of thrust. Since only a limited amount of air can be run through the engine, the effect of water vapor is to reduce power available.
This loss of power available and afore mentioned cooling effects tend to offset each other in typical helicopter operating scenarios, so engine performance doesn’t change as much with humidity (Figure 1-12) as it does with temperature (discussed in Chapter Five).
Furthermore, with respect to rotor systems, reduced air density decreases the lift produced on the rotor blades as discussed further with the lift equation in Chapter Two. For this reason, the overall effect of humidity degrades helicopter performance.
The previous philosophy on how to handle the effect of moisture on DA varied from either no adjustment whatsoever to the 10% rule of thumb for relative humidity (RH), adopted by numerous NATOPS manuals, of adding 100 feet to your DA (based on PA and OAT), for every 10% relative humidity above 0% RH.
DA (corrected for RH) = DA (chart) + (100’ x RH/10%)
Note: The TH-57 NATOPS Manual refers to the 10% rule. Therefore, it will be used for test purposes.
The 10% adjustment factor stems from a linear approximation of the curve (Figure 1-13). Some manuals state that the relative humidity (RH) correction (10% adjustment factor) doesn't go into effect until the RH is above 40% (40% rule). This is because the same curve in Figure 1-13 can be estimated by two distinct slopes, with this 40% rule appearing to give a better approximation.
For example, 0-40% humidity results in no correction, and 50-100% humidity results in 100-600 feet altitude correction for humidity (rather than an errant 500-1000 feet correction) based on the
“10% rule”.
Figure 1-13 Thrust Variation with Humidity
A comparison of the three commonly used methods to compute DA indicates that a significant variation exists between the chosen methods. For example, if we assume an OAT of 30 C (86 F), PA of sea level and Dew Point of 30 C (RH=100%) we obtain a DA of 1800 feet based on the DA chart using PA and an uncorrected OAT. If we incorporate the "100 foot-10% RH rule" for 100% RH the DA is increased by 1000 feet to 2800 feet or to 2400 feet if we only apply the "rule of thumb" for RH above 40% as some NATOPS manuals dictate. The actual DA obtained from the DA chart using a corrected OAT yields 2200 feet.
Applied to typical aircraft performance data (CH53D), the effect of the differences is apparent in Figure 1-14. Making no adjustment might lead to overestimation of capabilities. The difference is small, but in situations where operations may be conducted close to safe power margins, consideration of all factors is important. Ignoring DA effects could mean that safe margins are exceeded. On the other hand, being overly conservative could impact the ability to complete a mission satisfactorily. The 40% rule offers the most accurate quick estimate of humidity effects, compared to the more conservative 10% rule.
DA (corrected for RH >40%) = DA (chart) + 100’(RH – 40%)/10%
And is only corrected for RH > 40%
Again, for calculations in the field, PA is easily obtained by setting 29.92" Hg in the Kollsman window of the barometric altimeter. Obtaining the dew point is usually a little more difficult unless you have access to a weather service. In the absence of any of this information, you can always assume a worst case scenario of 100% RH which is when the OAT and dew point are the same.
For comparison purposes, Figure 1-14 below depicts for a CH-53D the relative accuracy of the three techniques for humidity correction of DA with an RH of 100%.
Adjustment DA Max Gross Wt (HOGE)
NONE 7800 FT 34,800 lbs
10% RULE 8800 FT 34,000 lbs
DA COMPUTER 8400 FT 34,500 lbs
40% RULE 8400 FT 34,500 lbs
Figure 1-14 Sample Effects of DA Calculations on CH-53D Performance
The 10% Rule provides a more conservative estimate of DA and is the recommended method in most helicopter NATOPS manuals. If a NATOPS manual does not discuss the effects of humidity, be conservative and apply the 10% Rule.