11
$\begingroup$

A pilot of a high-performance airplane should be aware that flying a steeper-than-normal VASI glide slope angle may result in:

A) a hard landing

B) increased landing rollout

C) landing short of the runway threshold

This is an ATP question and the answer is B. Could anybody explain why the answer is B, not A?

Improve this question
$\endgroup$
1
  • 4
    $\begingroup$ It'll be A) if the aircraft is not flared, won't it. $\endgroup$
    – Koyovis
    Commented Sep 7, 2017 at 4:24

4 Answers 4

Reset to default
13
$\begingroup$

I'll just formalize my comment here as an answer:

Landing is all about managing the aircraft's energy. When you are coming in for a landing with a steep approach it usually means that you will have more energy (speed) as you approach the ground. A high ground speed as you approach the runway usually means floating and/or, just like in a car, a longer distance to slow down. That extended distance you would need to bring the plane to a stop or slow enough to taxi off the runway would be the extended roll-out. Therefore B is the best answer.

A is incorrect because a steep approach does not necessarily mean a hard landing. The smoothness of a landing is dependent on the pilot knowing the appropriate moment to flare.

C is incorrect because unless the pilot is trying to land short of the threshold a steep approach can be done anywhere over any part of the runway/approach.

Improve this answer
$\endgroup$
5
  • 3
    $\begingroup$ Why would you be faster? Is it for safety reason? And is it actually included in typical Vref calculation? $\endgroup$
    – Jan Hudec
    Commented Sep 19, 2017 at 18:21
  • $\begingroup$ Unless your aircraft has an minimal L/D in landing configuration (including air brakes) such that the increased glideslope results in a higher speed than Vref with engines idle, I see no reason for a higher landing airspeed. As for C, the pilot is following the VASI, which guides the pilot to a fixed point on the runway (excluding flare). Under the assumption that the pilot follows the VASI, they have no choice where to land. $\endgroup$
    – Sanchises
    Commented Sep 20, 2017 at 10:43
  • $\begingroup$ @JanHudec from a pure physical point of view energy is retained. If you descend faster, more energy is converted from potential energy to kinetic energy. The only way to shed that energy is via drag. Drag is depending on velocity. So unless you are pretty fast, drag will not compensate for a larger speed increase. $\endgroup$
    – Adwaenyth
    Commented Sep 20, 2017 at 12:01
  • $\begingroup$ @Adwaenyth, remember, we are talking about powered approach. If you descend faster, more energy is converted from potential, so less energy is converted from fuel. $\endgroup$
    – Jan Hudec
    Commented Sep 21, 2017 at 6:17
  • 1
    $\begingroup$ @Adwaenyth, also, approaches are generally flown on the back of the power curve. Increasing speed decreases drag! This is because drag has the induced component, which decreases with (square of) speed and therefore there is certain speed at which drag is minimal. Extending flaps decreases that speed, but not below Vref in most aircraft. So you would have to be a lot faster to increase drag and you can't do that, because you must be slow for landing. $\endgroup$
    – Jan Hudec
    Commented Sep 21, 2017 at 6:21
8
$\begingroup$

A flare (or arresting of the sink rate) can't be initiated before the threshold / overrun area / displaced threshold because of say any protruding equipment such as light poles. That's why they included option 'c' as a trick answer, where aiming short is dangerous.

If the high sink-rate flare were to start at the same height / location and take the same duration as a normal flare, then the rapid increase in pitch would increase the angle-of-attack considerably and risk (or even cause) a stall. Resulting in 'a', which should not be the technique.

So the flare needs to be more gradual and to start higher, eating up more runway, answer 'b'.

enter image description here
(Own work) No overrun area or displaced threshold to benefit from.

a. same flare location / height, hard landing, risky technique
b. flare higher and more gradually, landing longer ✔
c. aiming short for same spot landing, risk of landing too short

enter image description here
(YouTube) Airbus A318 performs steep approaches—Vref is normal during this ~6° steep approach test / technique demonstration.

See how high the custom callout, "Standby... Standby... Flare," is commanded. Do note the overrun area at London City (first landing in the video), if it weren't there the landing would have been longer.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Actually, all three would risk a hard landing, better to round out a few feet above the runway, allowing speed to bleed off as you raise the nose and land. This is why a " round out " step is done, followed by the " flare " and " touchdown ". No need to be a hero and try for the " perfect " landing. $\endgroup$
    – Robert DiGiovanni
    Commented Sep 23, 2018 at 14:12
  • $\begingroup$ @RobertDiGiovanni - A few feet above the runway wouldn't work for the question, as it's for an ATP exam. Big airliners don't flare like light GA planes. $\endgroup$
    – user14897
    Commented Sep 23, 2018 at 14:17
  • $\begingroup$ Copy that. I would hope to have it together on short final, or go around. $\endgroup$
    – Robert DiGiovanni
    Commented Sep 23, 2018 at 14:20
1
$\begingroup$

Here is a briefing on this topic. A high performance Lancair 360 is used to show that energy state over the threshold is independent of rate of descent on final. Up to 2,000 fpm is possible in the Lancair. A video link in the brief shows this being demonstrated. http://www.n91cz.net/Operation/Landing_Energy_Analysis-web.pdf

Improve this answer
$\endgroup$
0
$\begingroup$

For a large and very expensive commercial aircraft, deviations from the glide slope as pictured would be potentially disastrous and potentially beyond the capabilities of the aircraft to safely land. One may wish to be right on that glide slope, especially if it was somebody else's plane. These would be go-arounds.

It is unfortunate that the "right" answer may be teaching us the wrong thing, and ignoring basic knowledge of how landing approaches can be flown.

An approach flown at flaps 30 will be steeper than flaps 0 with the SAME airspeed. The round out will be sharper, risking stall, however, it can be broken into 2 smaller rounds. The flare, touchdown, and rollout would be the same.

Really depends on what you fly, each aircraft has its own safe envelope.

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .