Proficiency: It’s not (just) the heat

According to FAA Aviation Weather,AC 00-6B, density altitude is the pressure altitude corrected for nonstandard temperature variations, and “the effects of humidity are generally negligible.” In contradiction, the FAA Safety Team’s Density Altitude publication (FAA-P-8740-2) states that the effects of humidity on density altitude can be significant, and “if high humidity does exist it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.” Unfortunately, the FAAS Team document ignores the effects a high density altitude has on regimes of flight other than takeoff distance and climb rate, and states, “humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency.” This is misleading because humidity isn’t the important variable affecting aerodynamic efficiency; water vapor pressure is. Water vapor pressure has an effect on density altitude, and therefore aerodynamic efficiency.

The reason water vapor decreases the density of air can be explained by understanding the composition of dry and moist air. Atmospheric dry air is 78 percent nitrogen (diatomic N2) with an atomic weight of 28 grams per mole, and 21 percent oxygen (diatomic O2) with an atomic weight of 32 grams per mole, for a total weight of 29 grams per mole. Water vapor (H2O) is less dense than dry air at 18 grams per mole, and its presence will therefore decrease the density of the air and increase the density altitude.

The Embry-Riddle Aeronautical University paper “Quantifying the Effects of Humidity on Density Altitude Calculations for Professional Aviation Education” in the International Journal of Aviation, Aeronautics, and Aerospace is an excellent resource for the pilot wanting to know more about the effects of humidity. Within its 37 pages is some complicated college-level math, but fortunately it provides a valuable rule of thumb for pilots wanting to apply a density altitude correction for the amount of water vapor in the air: Double the dew point temperature and add a zero.

The relationship between humidity, pressure altitude, and temperature is not linear and so the rule of thumb has two minor shortfalls. It should only be used when temperature is more than 5 degrees Celsius or 41 degrees Fahrenheit, which isn’t really a problem because at colder temperatures there isn’t much water vapor anyway and aircraft performance is less of an issue. Second, in some cases the rule of thumb can overestimate the humidity correction at sea-level pressure altitude by as much as 5 percent. The accuracy of the rule of thumb gets better as pressure altitude increases. It’s best to use an electronic flight calculator that has a dew point temperature input, but the rule of thumb will suffice for those who don’t have one. The rule of thumb may slightly overestimate at sea level, but overestimating is better than underestimating.

I fly helicopters in Southern Louisiana to offshore oil rigs, and summer dew point temperatures can reach 27 degrees C. The rule of thumb would add 540 feet as a correction to a pressure altitude/temperature-derived density altitude. This affects almost every performance parameter for the helicopters I fly. Those flying in high density altitude conditions may want to review their specific aircraft flight manual performance section to see how a high dew point temperature can affect their aircraft performance.

Many performance charts use pressure altitude and temperature on the two axes, respectively, instead of density altitude on one axis. In this case, just apply the rule of thumb humidity correction to the pressure altitude, and then apply to the performance chart.

Whether the effect of humidity is significant enough for a pilot to consider depends on the aircraft, dew point temperature, ambient air temperature, pressure altitude, and which performance parameter being considered. A helicopter may easily hover in ground effect with an ample power margin, whereas during an out-of-ground-effect hover the impact of humidity becomes significant. It’s a matter of understanding the various performance charts in the aircraft flight manual and the impact density altitude has when corrected for humidity.

Markus Lavenson is a captain in the Sikorsky S–92 and Leonardo AW139 helicopters.