Now that warm weather is finally returning our true airspeed is going up, and so is our landing distance.
We typically think of true airspeed only when planning the cruise portion of a flight. We need to know the true airspeed plus or minus the wind factor to know how long it will take to get to the destination. But have you ever considered how true airspeed affects your landing distance?
True airspeed is, of course, your actual speed through the air. True airspeed is indicated airspeed corrected for air density. Air density changes with temperature and pressure.
In flight a wing cares nothing about true airspeed. It is indicated airspeed that measures lift available. A way to think of this is that an airplane flies on indicated airspeed but we travel via true airspeed.
An airspeed indicator is really a pressure gauge measuring the difference between the pressure rammed into the pitot tube and the static free air outside the airplane. So the airspeed indicator is really a form of air density gauge. When the air is less dense because it is hot, or you are flying at a higher altitude, it takes more speed to ram that thinner air into the pitot to get the same indicated airspeed as you see with more dense air. The less dense the air, the higher the true airspeed for the same indicated airspeed value.
The wing is using air density to create lift so its lift generation is proportional to indicated airspeed. If all conditions are kept equal, the wing will always stall at the same indicated airspeed no matter what the true airspeed may be. Of course, aircraft weight and any acceleration above 1 G will change the indicated stalling airspeed.
Why does this matter on landing? Because on a landing approach we maintain the same target indicated airspeed for our weight no matter how dense the air is. The standard approach reference indicated airspeed is 1.3 times stalling speed for your airplane weight and configuration.
However, it is true airspeed that is a measure of how fast you are moving over the ground, not indicated airspeed. And that’s why true airspeed matters on landing and takeoff.
When you are approaching the runway on a hot day, or at a high elevation airport, you should fly the same indicated airspeed reference as you would on a cold day or at a low elevation airport. But when it’s hot and high that same indicated airspeed equals a higher true airspeed which means you are moving faster over the ground. When you touchdown after that hot and high approach your higher true airspeed equals a higher groundspeed so it’s going to take more runway to get stopped.
This is a really big deal in airplanes with higher approach speeds and greater weight, but hot and high landings will take more runway in any airplane.
Let’s say your airplane in landing configuration stalls at 50 knots. Applying the standard 30 percent margin gives you an approach reference airspeed of 65 knots indicated. On a freezing day approaching an airport at 3,000 feet elevation while maintaining the 65 knot target indicated airspeed your true airspeed is 67 knots. On a warm day at 35 degrees C your true airspeed with the same Vref indicated of 65 knots equals 71 knots true airspeed.
That extra four knots of true airspeed on the hot day means you cover about seven feet more every second on the hot and high landing than you did on the cold day. Float for a few seconds and lots more runway goes by. Since energy is a product of velocity touching down at four knots higher ground speed means the brakes have more energy to absorb to get stopped.
You can see how flying into the very high elevation airports in the west on hot days can stretch out your landing distance. But now that warm weather is finally returning, it’s useful to remember that any landing is going to require more distance than on those cold days many of us are trying to forget.