Climbing on VZF in a jet

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garfield
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Climbing on VZF in a jet

Post by garfield »

When you have a planned tailwind is it more time/fuel saving in a jet to climb all the way to your cruising altitude on VZF (low buffet)?
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Re: Climbing on VZF in a jet

Post by Jet Jockey »

We often cruise above the winds even when eastbound.
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Eric Janson
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Re: Climbing on VZF in a jet

Post by Eric Janson »

garfield wrote: Tue May 26, 2020 10:46 am When you have a planned tailwind is it more time/fuel saving in a jet to climb all the way to your cruising altitude on VZF (low buffet)?
We don't use VZF in large jets - it's called "green dot" and represents approximate minimum L/D. Not the most efficient way to climb - especially at the very high weights.

On my aircraft we have a cost index which will give 250knots below 10000' and then anywhere from 300KIAS to 320KIAS climb speed depending on weight then 0.78Mach until top of climb then 0.81Mach to 0.82Mach for cruise.

Flight Management Computer calculates an optimum altitude based on inserted data and this is where you can save a lot of fuel by climbing or not depending on conditions.

Example:- On a recent flight full fuel was the minimum required. Climbed with Cost Index 50 - cruise with cost index 0 and stayed as close to optimum altitude as possible. We didn't climb where the flightplan told us to but waited for the optimum altitude to increase. Managed 16+00 non stop.
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valleyboy
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Re: Climbing on VZF in a jet

Post by valleyboy »

250 below 10 grand is a joke - free speed should be an option given by atc
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telex
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Re: Climbing on VZF in a jet

Post by telex »

valleyboy wrote: Thu May 28, 2020 1:50 pm 250 below 10 grand is a joke - free speed should be an option given by atc
And 300 KIAS until 8 miles final!
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Re: Climbing on VZF in a jet

Post by valleyboy »

haha -- yup -- drive 'er like you stole her :smt040
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co-joe
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Re: Climbing on VZF in a jet

Post by co-joe »

This sounds like one of those goofy theoretical ATPL sample questions. You almost never climb for very long at Vzf or "min clean" it's just not efficient. It's well below best angle, best rate, and econ climb as computed by the fmc. The only reason to climb that slow that I can think of would be to minimise your turn radius climbing in high terrain.
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Fanblade
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Re: Climbing on VZF in a jet

Post by Fanblade »

garfield wrote: Tue May 26, 2020 10:46 am When you have a planned tailwind is it more time/fuel saving in a jet to climb all the way to your cruising altitude on VZF (low buffet)?
Not that slow as others have pointed out. High altitude, too slow = backside of the power curve = Your going down, not up. The heavier you are the less wiggle room you have for speed changes at high altitude. For this reason most aircraft transition to their cruise Mach during climb. If you can cruise at it, you can climb at it safely. It is usually the IAS prior to transitioning to Mach that is modified for winds. For example. Climbing into a headwind the Speed might be 320kts, transition Mach .78. Climbing into a tailwind speed might be 250kts, transition Mach .78.

So the general concept of what you are asking? The answer is yes. In modern aircraft climb and decent speeds are calculated by the FMS. Climbing or descending into a tailwind the computed speed will be slower. Headwind faster. Of course we also fly cost indexes. Much of this calculation is wiped out at high cost indexes. High cost index = barber pole almost always. On the other hand lower cost indexes = larger variations in climb/decent speeds due to wind.
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BTD
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Re: Climbing on VZF in a jet

Post by BTD »

co-joe wrote: Thu May 28, 2020 10:16 pm This sounds like one of those goofy theoretical ATPL sample questions. You almost never climb for very long at Vzf or "min clean" it's just not efficient. It's well below best angle, best rate, and econ climb as computed by the fmc. The only reason to climb that slow that I can think of would be to minimise your turn radius climbing in high terrain.
In the Max, Vzf or up maneuver speed approximates your best angle so you can use that if the FMS doesn’t give you an actual number for some reason.

The only time I’ve ever used a speed that slow in the climb is once or twice coming out of L.A. when they turn you enroute earlier than normal and there is high crossing restriction at one of your points not too far down the line. We just let ATC know we were going to be going slow. He was good with that as long as we made the altitude restriction.
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