True Airspeed and Landing Distance

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.

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16 Responses to True Airspeed and Landing Distance

  1. Glade Montgomery says:

    Nice description Mac. It makes complete sense.

  2. deadstick says:

    One word short. Lift is proportional to indicated airspeedsquared.

  3. I thought that was something everyone figured out before take off. Hah muvh runway do I need to get airborne and how many to land.

    I really had this pointed out to me on the way to FL. I left very cold temps in MI. I had to make a U-turn and land at Alma GA. because of the solid wall of fog just to the S of the runway. The next morning was one of days where you put your shirt on to dry it out. We were well under gross for the temp. I never saw so much runway go by before lift off, I had it figured correctly, but it still surprised me.

  4. Bill Tomlinson says:

    Isn’t every student supposed to know all this before he gets signed off for a PPL and the right to carry passengers?

  5. J.G.Sayle says:

    I don’t believe I’ve ever seen these basic fundamentals set forth so clearly in so few words.

  6. Thomas Boyle says:

    Succinctly put, Mac!

    A few words, though, from the slow end of the spectrum. A great deal of what’s written about flying the approach – including this blog entry – is written from the perspective of fast airplanes, for which runways are short. This is indeed an important concern: pilots who “grew up” on slow airplanes, tend to become lazy about airspeed and landing distances, and feel good about “adding a few knots for the kids” on the approach. When they take this kind of thinking to faster, heavier aircraft, they wind up overrunning the runway. And so, instructors have learned to hammer home the message: don’t fly too fast on the approach.

    However, for slow airplanes, like LSA, in practice most paved runways are effectively infinitely long. On the other hand, ordinary wind gusts are a large percentage of flying speed, and treeline rotor turbulence can rapidly overturn an aircraft with a wing loading well south of 10 lb/sq ft. The greater danger, for lightly-loaded, slow-flying aircraft, comes not from flying the approach too fast, but from flying it too slowly. For these airplanes, adding not just a few, but as much as 10-15 knots “for the kids” is entirely appropriate most of the time.

    The situation is more extreme for ultralights, of course, where wing loadings can be below 5 lb/sq ft., and stall speeds around 25kt (so that 1/3 x Vs would 8kt of stall margin).

    So, by all means, take care not to let the airspeed drift up when landing your PC-12. But do take care to keep it up when landing your RV-12 (on most paved airport runways).

    Light airplanes really are different!

    • Bill Tomlinson says:

      The best advice I was ever given for dealing with gusty crosswinds, treeline rotor turbulence and he like was to keep the power on until the wheels touch. Makes an amazing difference.

      • Ken K. says:

        Bill, try to learn not to use power as a primary flight control. Use the stick and rudder for that – that’s what they are for. If that means occasionally applying more than “a little” control input, so be it. I was taught – don’t count on “power” to save your butt.. instead, learn to fly the airplane.

  7. Bill Tomlinson says:

    Hi Ken. Thanks for your response, but I think you have misunderstood what I was saying. I most certainly would not advocate using power as a primary flight control.

    Most pilots either make a glide approach or, when making a power approach, pull off the power at hold-off height. What I am saying is, once you have turned finals and stabilised your glide-path, do not touch the throttle until you hear the wheels rumbling on the ground.

    • Ken K. says:

      I haven’t found that flying a small airplane onto the runway with power is ever a good idea. Maybe for something large and heavy, but for a small, light aircraft it’s just bad technique.

      • Once I’ve started down final, most of my flying is by feel. I pay very little attention the the gauges except for quick glances. They will almost always be where I expect.
        To me, a crosswind landing may be any where within the limits. If I can’t keep it straight and the flight path aligned I’d better try a different runway, or hope “it gets better” before I run out of gas. If that even appeared to be a possibility I’d go to one of the neighboring airports.

        My final landing for my Instrument check ride was with the rudder against the stops. De remarked, “Nice Landing”. That was right after the tower gave me a circle to land, right in front of a departing jet.<:-)) The weather was bad enough I'd considered canceling. Once there the DE said, "Lets play it be ear"..Other than getting the snot beat out of us the check ride went fine.

      • Chris M. says:

        That is unless you’re doing a wheel landing in a taildragger… a touch of power just before touchdown smoothes out the sink rate and helps keep you straight, particularly in a crosswind.

        • Land ‘em all the same way. After the third landing the ASF instructor asked me if I learned in a tail dragger, Mains are rugged, nose gear is fragile, expensive and for steering on the ground. Keep it off the runway as long as possible. Makes smooth landings and brakes last a long time. Uses a surprisingly small amount of runway too.

          • Doug D says:

            Works on wheels. Not so much on floats. On glassy water better be doing a power approach and pull it after touch down or you will most likely be swimming. Same on rough water. Back of the floats are weak. Need to keep the impact on the keels. There is almost never a never and never an always on water. Except that the runway is never the same. It’s always different.

    • My final in a Beech is almost constantly slowly backing ogg the throttle annd adjusting trim. It’s all automatic.

      Those great big barn door flaps Flaps are always full by the time the wheels are ready o touch and the nose up trim at that point is full up. That’s one of the signs I did it right. Even in gusty crosswinds at the airplanes limit of 25 knots, I’ve never needed less than full flaps. Less flaps means more float, until no flaps gives you a plane that does not want to set down on a very long runway. The Bo is a very good short field airplane, if flown by the book. As they told me in the ABS/ASF Bo specific training, there are very few Bo pilots who do not land too fast. Final in my old deb with the IO470N (260HP) and all the speed mods was about 75 mph (not knots) At night I flew final at 80 to give a more gradual descent. I rarely flew a stabilized pattern. I wanted to be able to set down on a designated spot, with someone directing wheels down, base turn and final turn and still hit that designated spot like Oshkosh!

      Even with gusty crosswinds it was rarely necessary to fly faster than 85. The planes limit Max rudder/ailerons/power limits and mine were the same. (25 knots) Even a tad more meant a goaround. The same on takeoffs, other than it meant an aborted one. Take it all the way to max and discover you still didn’t have enough Rudder/aileron and HP to keep it straight got exciting in a hurry as I found out at Oshkosh with an abrupt wind change from 270 to 180. We were skidding sideways with no more power available. I was able to pull it off into ground effect with the plane slewing a good 45 degrees. The N-40 looked like a prairie dog town with all the heads popping up. The Ford Tri-Motor was landing right behind me. When he saw that, Cody opted for Pioneer Airport <:-)). We cleared the highway by less than 100 feet. We were in the clouds from around a 1000 to 7000 and at 7000 it was like a sauna. We crossed Lake MI at 8000 for traffic avoidance. At 8000 it was nice and comfortable.

      The point is, as the wind changes with the reduction in altitude, you had better be ready for a go-around even after doing everything right be it gliding or powered landing. At 75 mph in a Bo you were carrying a lot of power. It's just not your primary control.

  8. Bill Tomlinson says:

    Sorry Ken, but I disagree. I have always found it works well, and I was based for some years at a field that had these problems in a big way.

    As for “applying more than “a little” control input”, most of my flying is competition aerobatics, so I am well used to putting the flight controls “into the corners”.

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