Flap retraction on landing

[ted6357]

On light singles [e.g. C152/C172 etc.] and perhaps light twin engine aircrafts, do you think it is good practice to retract the flaps soon after touchdown on landing as an aid to dump/kill the lift? [the equivalent of raising the spoilers on larger aircraft].

 

[Jim Logajan]

And there are very, very few light planes where the takeoff roll is shorter than landing roll with flaps left extended, so there is almost never anything to be gained by retracting the flaps on landing unless you are planning to have the plane trucked out of the landing site.

I used to agree with the above until I carefully read the Cessna PoH for the 172 and 152 when I started to actually plan landings at short grass fields with both airplanes.

The landing (and takeoff) distance tables come with a number of preconditions, such as "Maximum performance technique as specified in Section <whatever>." One of the performance techniques Cessna mentions in short field landings is retracting the flaps to maximize braking. It doesn't say how much to extend the landing distances in its table figures if flaps aren't immediately retracted, so one is advised to do so if there is a concern.

Example, using figures from the C-172M PoH:Sea level takeoff
Calm winds
20 C
lightly loaded (1900 lbs)
On grass.
Takeoff:580 ft ground roll, increase by 15% per notes (87 ft), yielding 667 ft.
1035 ft to clear 50 ft obstacle, with added ground roll, yielding 1122 ft.
Landing:530 ft ground roll, increase by 45% per notes (239 ft), to 769 ft. Longer than the takeoff run of 667!
1265 ft to clear 50 ft obstacle, with added ground roll, yielding 1504 ft. Longer than the takeoff distance of 1122 to clear.
OK - I cheated a little bit in my scenario selection (but only because it was actually relevant to my flights!) A lightly loaded C-172 has moderately decent short field takeoff performance, so the takeoff distances become longer or comparable to the landing distances when landing on grass. Otherwise takeoff distances for heavily loaded machines on paved runways are the limiting factors, not the landing distances. Flaps up for braking in such cases may not be needed.

By the way, the same is true for the C-152: its ground rolls also stretch out by 45% on grass. In a number of cases that makes the takeoff distance shorter than the landing distance.

 

[Jim Logajan]

I think, based on my own experience (applies to C-172s), if you are seeing a big difference by retracting flaps you are touching down to fast to start with. If you're really dealing with a short field landing you should have been on the back side of the power curve before touching down. Pull the power and you are DONE flying.

Not done rolling, though. The only way to know whether there is a difference worth worrying about in the braking distance is to fly some experimental landings by a pilot who is otherwise consistent approach speeds, the winds don't vary much during the test runs, and there is some way to measure the stopping distance without a lot of hassle (a camera might come in handy for immediate recording of location landmarks.) Half the landings with flaps left in, half with flaps immediately retracted; maybe alternate the landings.

If the weather were better here I'd be tempted to try it. Could use the practice anyway.

 

[Jim Logajan]

Hit your speeds right and don't come in faster than book speed and the plane will do what you want it to do.

I want it to come a-waltzing Matilda with me.

 

[Jim Logajan]

Lord forgive me, but I've also been known to turn off carb heat immediately after landing. Paired actions:

"Push" (carb heat)
"Flip up" (flap switch)

My pseudo-religious incantation on landing - either immediately or as I'm turning off the runway. Rare for me to do one without the other.

 

[Jim Logajan]

Hmm, you slow down faster with flaps retracted? Less low-speed lift with the flaps up = more friction with the ground? Before reading this my assumption was always that they would add drag and slow you down faster.

Seeing as how I'm fixed gear is this a good short field procedure to have in my back pocket?

Some technical background from here:

http://adg.stanford.edu/aa241/performance/landing.html
Although flap drag plays a significant role in the air run, the pilot's control of the throttles is: usually more important. If more power is maintained during the air run, the effect is the same as a higher effective L/D ratio. Furthermore, the touchdown speed is important since the wheel brakes are much more effective in retarding the airplane than the air drag during the air run. The sooner the airplane touches down and starts braking, the shorter the total distance will be. Thus,the human factor plays a large role in landing distances. The official landing distance is partly a reflection of how hard the flight test pilot worked to optimize the landing. In practice, this is dependent on how important the landing field length is to the usefulness of the airplane.If the landing distance is much shorter than the take-off distance, a little longer flight test landing may not be detrimental.

Mechanical devices have a large influence on landing distances. Automatic spoilers are operated by the rotation of the wheels at touch-down. The spoilers greatly decrease the lift, dump the weight on the wheels and thereby make the brakes effective. Manual spoilers, operated by the pilot, involve a delay. Even two seconds at speeds of 200 ft/sec. can increase the stopping distance by almost 400 ft. Including the safety factor of 67%, the effect on the field length can be close to 600 ft. With one exception, the curves on the figure are for automatic spoilers. In the 747 example on the chart, manual spoilers are shown to cost 400 ft. in field length. The adjustment of anti-skid braking systems can also affect the average braking coefficient of friction during the deceleration.

 

[Jim Logajan]

It appears there are two camps.

Which camp is serving the better food and drink?
I'll join that camp.