In these hot and humid weather, we thought we would remind you of the effect of density altitude on the performance of your aircraft.
The performance of your device depends on airport altitude, temperature and humidity. If these parameters increase, the lift, the rate of climb as well as the power of your engine will be less. The only thing that increases...is your stall speed. So in hot and humid weather you lose on all counts.
Density altitude is pressure altitude corrected for temperature. Take for example, a Piper Cherokee 140 at Thedford Mines (elevation 1,408 feet), with an atmospheric pressure of 29.75” Hg, the pressure altitude is 1,566 feet:
- at 20 C, density altitude = 2,510 feet
- at 35 C, density altitude = 4,176 feet
At 35 C this plane will behave as if it were at 4,176 feet:
- the take-off run increased to 1,083 feet. A 12% increase over 20 C conditions
- the distance to clear a 50 foot obstacle is now 2,361 feet. A 13% increase over 20 C conditions
- the rate of climb decreased to 492 feet/min. A decrease of 12%, again compared to conditions at 20 C
These numbers are taken from the Cherokee operations manual. They were established from a new aircraft and a new engine. It is certain that an aircraft of the same type no longer has this kind of performance today.
Our best advice is the 50/70 rule. If you have not reached 70% of your takeoff speed at 50% of the available length of the runway, you should abort your takeoff.
Watch this video which demonstrates very well the dangers of neglecting the degradation of the performance of an aircraft in high density altitude conditions. Click HERE
Adapted from a text by Gilles Jean.
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