Heavy Aircraft ToD ?

[WaldoPepper]

Why does a heavier a/c descend earlier ?

Is it due to increased momentum ?

[jtr]

Talking in jet speeds, the main contributing factor will be that the arbitrary 300kt descent means the heavy a/c is closer to best glide speed than a lighter one. To a lesser extent, the energy value must come into play also. Standing by for the techie brigade to correct me.

[Bellerophon]

WaldoPepper

This topic keeps getting discussed every so often, and you will find many previous posts on it using the search facility. What follows is taken from a previous post of mine.

An aircraft’s best glide angle is found at max L/D, which does not vary with weight but is purely a function of the design and aerodynamic efficiency of the aircraft.

A variation in weight however does affect the speed at which max L/D occurs, the best glide speed increasing as the aircraft weight increases.

If we consider two aircraft, identical apart from their weights, both trying to achieve the best glide angle possible in their descents, we will find that the heavier aircraft, flying at its best glide speed (max L/D) will be flying faster than the lighter aircraft, which can still achieve the same glide angle, but only at a slower speed.

In practice, the best glide speed for both aircraft is likely to occur at too slow a speed for schedule regularity or ATC purposes etc., and so many operators have a fixed speed in descent for their aircraft, regardless of weight, and this fixed speed is generally well above the theoretical best glide speeds of either aircraft.

Let us assume that our airline requires us to descend at 300 kts IAS, and that the best glide speed for our heavier aircraft is 270 kts and our lighter aircraft is 250 kts.

Both could achieve exactly the same glide performance if allowed to fly at their varying best glide speeds, but they have both been made to fly at the same speed.

In this example, at 300 kts the heavier aircraft is much nearer to its best glide speed (only 30 kts too fast) than the lighter aircraft (50 kts too fast).

Being closer to its best glide speed will mean a better glide performance, and so the heavier aircraft will lose less height over a given ground distance than the lighter aircraft.

As both are flying at the same airspeed, this means that the heavier aircraft has a lower rate of descent than the lighter one, and will therefore have to start its descent earlier.

Regards

Bellerophon

[Keith.Williams.]

Waldo,

Does your question refer to the rate of descent in a glide as discussed above, or to the time taken to commence drift down following a single engine failure?

There is a JAR ATPL Performance exam question relating to the latter option. The answer to this question is that an aircraft will generate more drag when heavy than when light. So there will be a greater thrust deficit following an engine failure when heavy. This will cause a more rapid deceleration to drift down speed, at which point the drift down commences. So an aircraft will start to drift down sooner when heavy than when light. If you are studying for the JAR exams you can see this effect in the drift down profiles on pages 86 to 89 of the CAP 698.