Classic Interview/Tech Question
Thread Starter
Join Date: Jun 2007
Location: Northern Hemisphere
Posts: 19
Likes: 0
Received 0 Likes
on
0 Posts
Classic Interview/Tech Question
I know this one has been done to death on the tech log but just wanted some more input to clear my head! 
"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.
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?
Having a difficult study session tonight. Maybe a G&T would help!
Anyone
Thanks
WC
"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.
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?
Having a difficult study session tonight. Maybe a G&T would help!
Anyone
Thanks
WC
Join Date: Jan 2006
Location: here, there, everywhere
Posts: 279
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Apr 2007
Location: West Coast, USA
Age: 50
Posts: 10
Likes: 0
Received 0 Likes
on
0 Posts
funny this should come up...
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
Yellow
Join Date: Jan 2006
Location: Scatman's World
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jan 2006
Location: here, there, everywhere
Posts: 279
Likes: 0
Received 0 Likes
on
0 Posts
The glide ratio of those 2 aircraft remain the same despite of the different weight
Geez, such a simple question, so many different explanations
Join Date: Jun 2007
Location: A Red State
Posts: 33
Likes: 0
Received 0 Likes
on
0 Posts
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.
I always knew this was true, but explaining the theory behind it got me a little foggy -- thanks for the discussion.
Join Date: Mar 2007
Location: Aus
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Nov 2004
Location: 41,000'
Posts: 279
Likes: 0
Received 0 Likes
on
0 Posts
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.
