Heat and aircraft performance

Why heat changes aircraft performance

Sweltering temperatures throw off the balance of basic physics for aircraft performance. Technically, it is not the heat itself that matters but the effect of increased temperature on air density. The Federal Aviation Administration (FAA) discusses this in safety guidance, which formally defines density altitude as “pressure altitude corrected for non-standard temperature variations.” Essentially, as air heats up, it expands and becomes less dense. Density altitude is the pressure altitude adjusted for temperature.

Hot air cuts its own lift. The FAA notes that when temperatures rise above average for a location, the air density is reduced and the density altitude increases. This in turn lowers an aircraft’s lift and performance. Concretely, advisors put it: every 1,000-foot increase in density altitude causes normally aspirated single-engine aircraft to see a 3% drop in power and a 15% additional increase in needed takeoff distance, studies show.

An assortment of flight mechanics can trip up an aircraft in the heat: the takeoff and landing distance are affected; climb performance and cruise speed go down. All of these add up in the summer desert heat — and mean that a flight operation has less margin for error.

The Runway Becomes A Bottleneck

The pragmatic consequence of physics has airlines waiting to play their next card. When reduced performance meets a runway that is its own bottleneck, takeoff performance suffers the most. Airport operators now need more runway to meet normal operational targets.

Even though a runway might be approved for certain use, those are calculated for standard conditions. As the atmosphere warms, the actual conditions drift from what was the norm. Remember: density altitude raises the effective height of the airfield.

The FAA goes into this in another document from this year, Aviation Weather Services: it says that the takeoff roll, climb-out, and landing distances are increased as temperatures climb, in part by raising the density altitude. Taking off in the heat now becomes a double squeeze, where the runway becomes too short for the need, even though it remains technically approved for the aircraft.

When You Have To Drop What You Bring

Operations need to rise to the challenge by dropping their load. The plane itself becomes the true burden on the performance. For the same reasons density altitude works on flight, the effects of temperature and altitude also constrain what an aircraft can carry aboard.

U.S. weather experts point out that operators, including general and private aviation, ought to avoid full-load departures “close to the margins” if they are trying to lift-off at a high density altitude, in guidance from the Aviation Safety Reporting System [a NASA-run initiative that logs safety data and offers guidance].

The payoff from those precautions is calculated in the same domain as the problem: the complex issue of extra needed runway on hot days. Again, the distance increases. The once-certified runway may not provide the extra lift and ground speed to clear obstacles, something the pilots have to check.