# How does deploying full flaps affect Takeoff run and distance?

There is a question in EASA Question Bank (AviationExam) that asks:

How will the takeoff distance change if the flaps are increased from 0 to full?

(a) Reduce (this is marked as correct since many student have reported this option as being correct from their exams)

(b) Increase

(c) No Change

(d) Reduce until medium setting, then increase

My understanding was that after 15/20$$^{\circ}$$ further flaps deployment is only for increasing drag and slowing down the aircraft.

Here is an excerpt from CAE Oxford Principles of Flight Book:

Take-off distance depends upon unstick speed and rate of acceleration to that speed.

a) Lowest unstick speed will be possible at the highest CLMAX and this will be achieved at a large flap angle

b) But large flap angles also give high drag, which will reduce acceleration and increase the distance required to accelerate to unstick speed.

c) A lower flap angle will give a higher unstick speed but better acceleration, and so give a shorter distance to unstick.

After take-off, a minimum climb gradient is required in the take-off configuration. Climb gradient is reduced by flap, so if climb gradient is limiting, a lesser flap angle may be selected even though it gives a longer take-off distance

The above (TOR and TOD both increase) seems to contradict the answer EASA considers correct...

This question is similar to Should full flaps be deployed on takeoff? and accepted answer is more or less saying what the oxford book is saying. So my question is:

Is there any scenario or plausible logic that might make EASA answer correct for even a single corner case?

• D was obviously the answer intended by whoever originally composed the question. But, that's not a full answer to the question you are actually asking here-- Commented Dec 20, 2022 at 15:38
• There are very often errors in question banks used by online study guides - I've seen this reported time and time again over the years from student pilots on various online forums. D is quite obviously the correct answer.
– Jamiec
Commented Dec 20, 2022 at 16:01
• Also (not surprisingly), the question is ambiguous. In option D, in the phrase "and then increase", is that simply supposed to mean a progressive increase with each extra degree of flap, compared to with one degree less flap? Or does that mean that the takeoff distance actually becomes greater than with zero flap? If the latter, that statement is obviously false if the flaps are deployed only slightly more (say one degree more (say one degree more) than the setting that gives the min. takeoff length, and arguably in some aircraft designs (ctd) Commented Dec 20, 2022 at 17:18
• (ctd) and arguably in some aircraft designs (say with limited total flap availability, e.g. later-model Ces 152 w/ 30 degrees max flap available) might even be true in some conditions (e.g. muddy field) even with full flaps. So depending on how we read the question, an argument can be made that in some cases option A could be the best answer. Commented Dec 20, 2022 at 17:19
• It depends on the $C_l$ an zero flaps and that at full flaps along with many other variables such as engine thrust, their propulsive efficiency and even $C_d$ just to name a few. So the correct answer is going to vary from aircraft to aircraft - there is no correct answer. However, option D seems to be true for most aircraft. Commented Dec 20, 2022 at 23:15

Is there any scenario or plausible logic that might make EASA answer [meaning answer A] correct for even a single corner case?

Yes-- consider a Cessna 152 with 30 degrees maximum flaps available, on a soft muddy field, with no wind. The takeoff distance will be less with full flaps than with no flaps.

The takeoff distance might be slightly less with less flaps-- say 20 or 25 degrees flaps-- than with 30 degrees flaps, but that would not be inconsistent with answer A still being correct for this particular case.

Answer D is arguably a good answer too, depending on how we read it-- it's not clear whether the phrase "and then increase" is supposed to mean "and then becomes larger than the takeoff distance with zero flaps", or is supposed to mean "and then becomes larger than the takeoff distance with somewhat less flaps." If we understand the answer to mean the latter, then "D" is arguably the best answer, because it would be true for nearly all aircraft in nearly all situations, unless the flap travel is extremely limited-- after the flaps have been deployed to some optimum value, further deployment will usually start to increase the takeoff distance again, even in the case of a soft field, etc. In contrast, if we read the phrase "and then increase" to mean "and then becomes larger than the takeoff distance with zero flaps", then this is true most of the time, but it's not difficult to think of exceptions, e.g. the case described in the paragraph immediately above.

It seems almost certain that answer "D" was intended to be the correct answer.

• Thanks. I am quite sure exam questions are supposed to be for general or common situations, and if the examiner had soft field TO with C-152 example in mind, he would have specified. Who ever wrote that question probably had option D in mind but somehow it got messed up when fed into QB. Several students had reported option A as correct answer by EASA, so I am quite certain the problem exist in EASA QB and AvExam has replicated that issue in their own QB. Commented Dec 21, 2022 at 9:33

Yes, (d) would seem far and away the best answer unless:

The aircraft had a very high thrust to weight ratio. Then (a) is also a possibility.

It is very important to know the limitations of each type of aircraft.

The strategy for lift-off in as short a distance as possible is to generate adequate lift from improving Coefficient of Lift and airspeed, which factors in as V$$^2$$.

Thrust vs drag and mass are the deciding factors.

An aircraft with a very low thrust to weight ratio, such as a powered glider, benefits from keeping drag low and even more from a rolling start. Lift-off airspeed difference flaps up and down may be as little as 5 knots.

A more powerful aircraft, such as a C-130 J transport, with prop blast under the wing and plenty of thrust to move, might do better with flaps down.

It is important to remember that flaps increase the AoA of the part of the wing where they are deployed. Climbs with flaps can be done with the nose lower to the horizon as long as an altitude gain is seen. Slats and flaps can add more camber to the wing without significantly changing local AoA.

Reducing takeoff distance with full flaps only is rare. However, a soft field take off$$^1$$, where the ground surface adds to the drag, may favor a higher flap setting.

$$^1$$ as suggested in comments