Many type-certificated aircraft, especially Transport Category aircraft, have a landing reference speed (Vref) that is equal to 1.3 times the stall speed. Some owners of amateur-built aircraft believe that they can select an appropriate approach speed by simply noting the indicated airspeed reading at the stall, and multiply this by 1.3. This approach is not appropriate in many cases.

Many aircraft have significant airspeed system errors at low speed. A landing reference speed (Vref) of 1.3 times the stall speed only provides the expected margin from the stall if it is based on calibrated airspeeds. I.e multiply the CAS at the stall by 1.3, and then fly at this CAS. Many airspeed systems under read at the stall. If the IAS at the stall is multiplied by 1.3, one may be much closer to the stall than expected. For example, the POH for the Cessna 182Q that I fly once in a while says the max weight, aft CG landing configuration stall is at 50 KCAS or 38 KIAS. If we fly at 1.3 times 38 KIAS, that is 49 KIAS. The position error chart says that 49 KIAS = 55 KCAS, or 1.1 times the stall speed. Anyone who tried to fly an approach at 49 KIAS would likely get a nasty surprise when they tried to flare.

It is quite unlikely that very many amateur-built aircraft owners have the means to determine the calibrated airspeed at the stall. So there is no practical way to determine an approach speed that is 1.3 times the CAS at the stall. What is a fellow to do? I recommend the following, based on the flight tests that are used to determine minimum safe approach speeds for both light aircraft and transport category aircraft:

  1. Fly the aircraft enough to have developed a consistent normal approach technique.
  2. Ballast the aircraft to max landing weight at forward CG. Make sure the ballast is well secured.
  3. Climb to a safe altitude, and conduct a series of simulated approaches and landing flares, using the same technique as you would use in a real approach and landing. Reduce the simulated approach speed by a knot or two each time. Note the aircraft controlability at the approach speed, and note the ability to flare. Note the minimum speed where the aircraft has satisfactory characteristics.
  4. Pick a day with very light winds and no turbulence or wind gusts. The best time to find such conditions is right after sunrise, but even then there may be several weeks between days with suitable conditions. Be patient. Don't risk a hard landing by doing these tests on a day with gusts or turbulence.
  5. Fly a series of approach and landings, using your normal approach and landing technique, but reducing the approach speed by a knot or two each time. Pay attention to how the aircraft responds to the flight control inputs in the flare. Note any signs of inadequate control, impending stall, difficulty in obtaining a satisfactory touchdown, etc. As you reduce the approach speed, eventually the aircraft will start talking to you. It will tell you that you shouldn't reduce the approach speed any further, or you will risk a hard landing, etc.
  6. Caution - Don't be too aggressive about trying to demonstrate the absolute minimum possible speed. This path leads to a hard landing, and possible aircraft damage. Stop the investigation when you have a slow minimum speed that allows an acceptable flare and touchdown, using your normal technique.
  7. The following restrictions, from the requirements for type-certificate aircraft, are recommended to ensure that a minimum speed approach and landing demonstration is not a "party trick":
    1. Once the power has been brought to idle, there should be no need to increase the power. By all means, if you need to increase power to achieve a safe touchdown, increase the power. But you should conclude that perhaps the approach speed was too low, and the minimum approach speed should be increased.
    2. The power changes that are made should be the same as would be made during a normal approach and landing. E.g, if a normal approach and landing has the power brought to idle in the flare, it should be possible to do the same during the demonstration of the minimum approach speed.
  8. Add a comfortable increment (perhaps 5 kt) to this minimum demonstrated approach speed, and this becomes your minimum operational approach speed, to only be used in calm conditions. If the conditions are not smooth, add a few more knots, so that as the airspeed bounces around in the bumps, the bottom of the bounces is no lower than your minimum operational approach speed.
  9. Confirm the ability to safely manoeuvre at your minimum operational approach speed by conducting turns at that speed at a bank angle that is a bit higher than the highest you would use in service. You should be able to maintain a stabilized turn without encountering stall warning. Increase the minimum operational approach speed if required to obtain satisfactory manoeuvring capability.
  10. If you have a short field landing technique that differs from your normal landing technique (perhaps you keep power on until touchdown, and use a minimal flare, etc), repeat the above series of tests using a short field technique. You may have different minimum approach speeds for normal and short field landings.

If you later change ASIs, or make any changes that affect static system errors or stall speed, repeat the above series of tests.