Classic Interview/Tech Question

[Wheezy Child]

I know this one has been done to death on the tech log but just wanted some more input to clear my head! :bored:

"If you are flying a heavier aircraft at the same altitude as a lighter one, do you have to descend earlier or later to arrive at the same point at the same time."

Here is my thinking.......

2 aircraft, one heavy, one light, start the descent at the same point, with the power at idle and say 300IAS for the sake of arguement.

Light has to push hard over to get 300kts and therefore has a very steep descent path and hits the ground say...20 miles later with many a Gin and Tonic spilt! :=

Heavy has a more gentle push to get 300kts as it has more potential energy and has a much shallower descent and therefore hits the ground after say 30 miles. :eek:

Therefore in answer to the question, the heavier aircraft must start down 10 miles earlier than the light for the Gin and Tonics to be spilt at the same point on the ground!!

Am I in the ball park or completely out to lunch with this? :ugh:
Having a difficult study session tonight. Maybe a G&T would help!

Anyone

Thanks

WC

[texas145]

yes, a heavier airplane must descend earlier than a lighter one, to make a crossing restriction, due to momentum.

[extreme P]

Heavier a/c needs a higher AOA. Adjust accordingly.

[ASH1111]

Bingo. This is a function of AOA.

[Wheezy Child]

Cheers Guys,

Thanks for all the input.

WC

[Stuck_in_an_ATR]

I don't think momentum has anything to do with that.

If you are talking about idle descent, then the heavier one will have to descend earlier as it has better glide ratio at high speeds (think about gliders with water ballast)

If you are talking about constant path descent, then obviosuly, TOD won't be a function of weight

As for momentum - it's only important when slowing down, not in a constant descent

[YellowFever]

I was talking about this today with my F/O (long flight, time to kill) and I really liked his version (Riddle Kid, not his fault...). He came at it from the "Potential Energy" side, more mass at altitude has more potential energy, as opposed to the momentum side, which is a little different. Because you are swapping more potential energy for kinetic (speed) you need more time to do it in... can also be explained in terms of AoA, as above, but I preferred this version.

Yellow

[Cheukert]

The heavier aircraft has to descent earlier to arrive at the same time and same point as the lighter one.
The glide ratio of those 2 aircraft remain the same despite of the different weight. However the heavier has a higher best glide speed and therefore can descent earlier.

[Stuck_in_an_ATR]

The glide ratio of those 2 aircraft remain the same despite of the different weight


The MAXIMUM glide ratio remains the same regardless of mass. If the two are to descend at the same (high) IAS, the heavier one will have a HIGHER glide ratio, than the lighter one...

Geez, such a simple question, so many different explanations :}

[texas145]

You are right, ATR, that's probably why they ask this question :)
I always knew this was true, but explaining the theory behind it got me a little foggy -- thanks for the discussion.

[Blogsey]

Just to drag up an old topic. Here's the best (and quickest explanation I can find: "Getting to grips with aircraft performance" www.smartcockpit.com

At a given TAS, a higher weight means that a higher lift coefficient (CL) is
needed to maintain the balance of forces. This is achieved by increasing the angle of
attack (α) and reducing the descent gradient (γ). As RD = TAS.γ, the rate of descent
is also reduced at higher weights.

[sizematters]

just follow the "chinese glideslope" on the FMC.............seems to work

[piston broke again]

Correct me if I'm wrong, but one light aircraft, one heavy aircraft at the same altitude...surely they would descend at the same point and maintain the same profile it's just that the heavier aircraft has more energy so it will maintain the profile at a faster airspeed.

[sizematters]

well the assumption was they both descend at the same speed


so that kinda makes you wrong, huh ??

[piston broke again]

SM,

Hence why I said 'correct me if I'm wrong'. Cheers.

PS - Never assume in aviation