Lightweight Aircraft During Descent, Approach and Landing
 

Hi Guys,

I've heard from many people that this forum is a good one to air a technical question.

I fly a twin-jet aircraft out of a major UK airport for a well-established carrier and the other day we were coming in weighing in at considerably less than normal, passenger load was about half what it normally is, and so the approach speeds were on the lower side.

The approach was interesting but it popped a question into my mind that, assuming no speed control by ATC, "Is it better to dive a light-weight aircraft and know it will lose the speed quickly in level flight?". My thinking revolves around the fact that it didn't seem to want to descend very well (the glide angle seemed shallower than normal) at light weights but it did seem to slow down quite quickly in level flight. Is there a best trade-off between keeping high speed (thus descending quickly) and not allowing the light aircraft to creep above the profile?

Thanks for your comments in advance!

Gusty

I'm perhaps not the best person to comment (being only a lapsed glider pilot) but nobody else as so...

My first thought was why did you allow the plane to dictate your speed? Perhaps it's different in a jet but in a glider you select a speed based on the aircraft type (eg it's recommended landing speed) and add an allowance for wind shear. Letting it fly slower/faster is not recommended. In a glider the elevator controls your speed and the air brakes/spoilers control your rate of descent/glide slope. Pointing the nose at the "aiming point" will get you there but too fast and you can't burn off the speed quickly because the plane is very clean. If you try and use elevator to slow down you get your height back.

As a general rule...a lighter aircraft can descend at a steeper angle for a given distance (I think this is right) purely because it has less potential energy. i.e. less mass

So the heavier you are the further you require to descend and slow down:ok:

(please ignore all of the above if Im wrong....it just seems to work that way in the 757's I fly:oh: )

yup on my airplane as well: less weight means steeper descent

simplified:

the composant of weight acting towards your flight path is added to your thrust (weight*sin angle)

if you take 2 planes at same angle of descent but different weight:

aircraft heavier will need more drag or will accelerate until drag=thrust
if both descending at idle and speed limiting (as Vmo) then the heavier aircraft will have to reduce its descent angle compared to the lighter one

Correct me if I'm wrong, thanks

A lighter plane does not glide a longer distance than a heavy, it only glides for a longer TIME. With lower weight the aircraft will have a lower descent rate AND a lower forward speed; thus the glide angle and distance is the same as with a heavier plane.

Hi Guys,

Thanks for your replies. I think you've helped me crack it. If I understand correctly, the feeling is that a heavy and a light aircraft will have the same glide angle if they fly at their optimum angle of attack. Right?

The only difference being that a heavy aircraft will require a faster speed to achieve this angle of attack, whereas a light aircraft achieves this best angle of attack at a lower speed.

Thus, a light aircraft descending at the same airspeed as a heavy aircraft will actually be diving at a steeper angle. Right?

This explains why my Training Captain said that it is better to dive off the profile in a light aircraft than to try and slow down. If I dive it off I achieve a steeper angle, then I can slow down easily in level flight. Whereas if I try and descend at a slower airspeed with my light aircraft I'll actually be coming closer to my best glide speed (or angle of attack), and actually sailing above the 'profile'...

I think it's becoming clearer and I'm no longer doing this :ugh: with my mind!

Any other thoughts on this?

Gusty

yes indeed, totally correct

This reminds me of the story:

Some controllers in the country I live had a tendency to slow down aircraft to have them descend steeper (they tought less speed, same RoD, must give greater slope) and kept doing that (not understanding why the aircraft would reduce their RoD) until an ex-pilot got a job as controller and gave them a perfo briefing :}

I think I'm starting to get a clearer picture but could someone just confirm that I am understanding this correctly:

- For a given airspeed (ie. Vmo) a heavier aircraft would have to begin descent earlier than a light aircraft because, due to the increased momentum, it would require maintaining a lower rate of descent to avoid exceeding this speed?

- If asked to maintain a slower airspeed than normal on the descent, the aircraft would have to descend earlier because it would then be descending closer to its best Lift:Drag angle of attack and would therefore have a lower rate of descent for the given airspeed?

Thanks in advance.

So far it is right...
Does it precisely follow?

Logically, you could dive at a steeper angle by flying at any angle of attack and airspeed other than the optimum angle of attack.

Thus, you might glide either at lower than optimum angle of attack, at a faster airspeed, or at a higher than optimum AoA, at a lower airspeed...

It's worth considering that with a lightish a/c the descent speed may be closer to Vmd and hence the glide is more efficient, i.e. burns off less energy to maintain the required speed. Given thrust is idle, the energy used up is mainly potential (height) energy.

Large aircraft have a lower recommended airspeed for landing the lighter they are. Small aircraft can get away with a one-speed-suits-all type approach, but that doesn't work on a larger scale.

My 2c on the matter. Drag is proportional to velocity squared... so.. for a specific configuration you can increase drag and loss of potential energy(altitude) by keeping the speed up.
This is for a specific configuration only.. putting out flaps/gear and spoilers will increase the drag above the clean configuration. A faster speed in those configurations will improve descent rate/energy loss as well.

Sounds reasonable to me.. feel free to poke holes in my thinking!

A light airplane can descend more steeply than a heavy airplane, but it doesn't have anything directly to do with momentum or acceleration.

Given the same airplane at 2 different weights, the speed for maximum glide will be lower for the light airplane, because the angle of attack for max glide will result in a lower airspeed. Remember that the glide ANGLE at max glide is the same, regardless of weight.

Take, for example, a B747. At max takeoff weight the max glide speed is around 260 KIAS; at light weight it can be as low as 210 KIAS. So, the lightweight 747 descending at 260 KIAS is descending much more steeply that the heavyweight, because it is flying 50 KT faster than optimum.

Your deduction is correct, but I think the reason is more "apparent thrust" than momentum. Your weight acts as thrust in the descent as explained earlier by JonaLX.

Yes and no. For the same rate-of-descent, your speed would be lower for a lighter a/c at idle power. In other words, you would need a higher ROD for the same speed, compared to a heavy a/c.

I practice, I plan idle descents for heavy approaches and power-on descents for a very light a/c. This keeps my ROD constant (start descent at same point) and I then use power to maintain same speed profile.

The worst case scenario in my a/c, is a light a/c in icing conditions (Min N2 limit with anti-ice) with Radar slowing you down early. You are almost guaranteed to need flight spoiler.

Am confused now.
.
Question for example, two 744s in cruise at same FL, one very heavy, one very light. Switch off the engines, which one will glide the longer distance in still air.???:oh:

I think they would glide the same distance since the glide angle (L/D) is the same.

This angle occurs at a higher speed on the heavier aircraft so it would get there faster. (Hence why racing gliders fill their wings with water!)

My 2ct worth

A lighter aircraft will glide as good as a heavier aircraft. Glide angle is defined primarily by the wing design: L/D ratio is equal for both a heavy and a light aircraft and can be considered a wing property. Best L/D ratio or best glide is achieved at a certain speed, this speed coincides with Vmindrag and is proportional to the stall speed. A heavy aircraft will obviously require a higher speed to glide the same angle as a light one.

However, in normal jet airline ops we never encounter this situation. For a normal descent we fly fixed speed (lets keep it simple). Descending at this fixed speed the heavier aircraft will have a better glide angle than the lighter purely because it flies closer to its "Best L/D ratio speed". Obviously the lighter aircraft will fly more away from this speed a.k.a. Vmd, which means a significantly higher relative drag, and thus descent angle.

If you want to see how weight affects the descent characteristics of your aircraft, you can play around with the FMC/FMGC Zero Fuel Weight. Just remember to put the correct weight back in the box when you finish!!!


GMS

Drag is proportional to velocity squared - but so is lift. If you keep the speed up at a specific configuration, your lift will increase above your weight, so the plane will accelerate upwards and lose rate of descent!

As long as the plane keeps a constant rate of descent, lift has to stay equal to weight. Therefore, you can only change your airspeed if you simultaneously change your lift coefficient, e. g. by changing the angle of attack.

Hi GMS,

Thats true. But the change in descent profile will be because of the ECON SPEED being further away from the optimum L/D (which is proportional to Vs1g) when being lighter weight. The ECON SPEED should be somewhat similar for both the heavy and the light cases.



Cheers