Approach speed

01 Jul 2006 Approach speed

Great comments from test pilot and RV8 builder Kevin Horton:

1.3 Vs works well on larger aircraft, where 30% of stall speed is a
fairly large number. But on a smaller aircraft, 30% of stall speed
might not leave enough margin to to deal with any turbulence on short
final (i.e. 30% of a 100 kt stall speed is twice as big as 30% of a
50 kt stall speed). Also keep in mind that the 30% of stall speed
used on large aircraft is 30% of CAS, not 30% of IAS. Many aircraft
have a large position error at the stall, so the IAS at the stall
might be quite a bit lower than the CAS. For example, the C182Q that
I fly once in a while has a stall speed at aft CG of 38 KIAS or 50
KCAS. 1.3 * 38 = 49 KIAS. 49 KIAS = 55 KCAS, or only about 1.1 Vs
(all values from the POH). If you tried an approach at 49 KIAS you
would have a horrible surprise once you tried to flare. On this
aircraft, the recommended short field landing speed is 60 KIAS, and
this seems about right to me. 60 KIAS is 1.58 Vs, with both values
in IAS.

As far as approach speed, keep in mind that you have unknown position
and instrument errors. So you can’t just take a recommended approach
speed from someone else and use it, or simply multiply your IAS stall
speed by 1.3 and believe that you have the magic number. The best
approach is to load the aircraft to max landing weight and forward CG
(the worst case). Pick a day with calm winds and no turbulence – you
want air as smooth as a baby’s bottom. Do a series of approaches and
landings, dropping the approach speed by a knot or two each time.
Note how the aircraft responds in the flare. Eventually you will be
slow enough that you start to have problems arresting the rate of
descent in the flare – the aircraft will be talking to you. Take
that speed and add a bit of margin, perhaps 5 kt. That becomes your
absolute minimum airspeed on final.

Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8