Landing flap question

[AeroSpark]

Enthusiast not a pilot so please excuse me if I'm asking something obvious. I would like to know why certain aircraft don't always use full flap on landing, I'm thinking in particular of the 737 which uses 30 deg of flap mostly? What are the advantages of this, and why wouldn't a pilot want to land as slowly as possible? Is flap 40 just used for particularly short runways, or in certain weather conditions?
Thanks in advance

[back to Boeing]

The list of reasons is quite varied. Im only au fait with Boeing. All the Boeing’s I’ve flown have 2 acceptable land flap settings in normal situations. 30/40 in the 737 family 25/30 in others.

a reduced flap setting uses slightly less fuel. Marginal on one approach but when compounded with several a day over a large fleet, it begins to count.

Also less wear on on the flap drive systems (screws have to turn less). But I’ve never found that in any official manual only old wives tales from engineers.

also the full flap setting has protections to prevent excessive forces if the speed is too high. So if you’re heavy and blustery you may be close to the flap limit speed for the full flap setting. If turbulent conditions force the indicated airspeed to go above the allowable speed the flap “blows back” to a lower setting to protect the wing. That will instantly make you “unstable” (a technical term who’s conditions im not
going to bore you with) which will necessitate a go around if youre too low on the approach.

another old wives tale (I say the term in jest because I have no way in personally telling which setting is which from the ground) is that a reduced flap setting is quieter. Less turbulence and less engine noise as the engines will be at a lower setting.

im sure others will add to the list but they’re the first I can think of off hand.

which setting to use generally comes down to SOP, conditions on the day, experience and personal preference of the crew.

[aterpster]

The 727 has landing flaps of 30 and 40. Early in the operation of the airplane a UAL flight made a very hard landing at KSLC that is resulted in an accident:

https://en.wikisource.org/wiki/Aircraft_Accident_Report:_United_Airlines_Flight_227

One of the conclusion was with 30 flaps the high descent rate approach might have been arrested. Because of this accident 30 flaps became the standard landing flap. Some airlines even blocked out 40 flaps.

[B2N2]

Aero, it’s an interesting question and the answer is not always that obvious.
Couple of possibilities just off the top of my head in random order.

1. Operator SOP, company prefers you land with flapsetting x for whatever reason. Their name is on the tail and their name is on your paycheck so you do as you are asked.

2. The 737 has gradually been stretched and stretched over the years. Now this doesn’t only apply to the 73 but to more aircraft: certain weights with certain flap settings increase or decrease the clearances for the tail during take off or landing.
Hence this weight or above this flapsetting.

3. Convenient/ no need to use full flap on a 12,000’ runway if you need to exit at “D” for minimum taxi time to stand 34.

4. At my company the bean counters have decided it’s cheaper and faster to replace brakes then it is flap components so the company recommendation is flaps 25.
I still use flaps 30 when we are very light as it’s simply fun to land a humongous plane at 120kts.

As an illustration to the possible forces and wear and tear:

https://m.youtube.com/watch?v=CjuiB8JPFC4

[AeroSpark]

Thanks all, very informative :ok:

[eckhard]

One reason for using the full flap setting is that it gives you a better view during low visibility landings. The nose attitude is slightly lower, resulting in a shorter distance from the pilots’ eyes to the first bit of runway that they can see. If they’re looking for 3 centreline lights, for example, it would be a shame if the nearest one was hidden under the nose!

[B2N2]

Thanks all, very informative :ok:


You’re welcome.
Thats why we are here.

[+TSRA]

One reason for using the full flap setting is that it gives you a better view during low visibility landings. The nose attitude is slightly lower, resulting in a shorter distance from the pilots’ eyes to the first bit of runway that they can see. If they’re looking for 3 centreline lights, for example, it would be a shame if the nearest one was hidden under the nose!

Unless restricted by the AOM or its supplements. The Q400 supplements in the AOM for CAT II and CAT IIIa do not permit a Flap 35 CAT approach. They're restricted to Flaps 15.

[eckhard]

Thanks for that. My only experience of auto-landing is in Boeing and Airbus.
Can you tell us the technical reasons behind that limitation on the Q400?

[Dan Winterland]

On the 747, moving flap 25 to flap 30, most of the increased deflection is taken up by the aft flap. This puts a lot of stress on it and subsequent increased maintenance. With the reasonably regular reports of flap sections breaking off 747s, it's invariably the aft flap that falls off. Some operators ask their crews to do flap 25 landings for this reason. However, the two airlines I flew the 747 with both had 30 as their SOP. The only exception was with the 747 freighter. At very light weights with a empty freighter, landing with flap 30 led to a very flat attitude with the increased risk of touching the nose-wheel first, so some pilots used flap 25 in these circumstances.