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trav110
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Tailwinds during takeoff

Wed Nov 30, 2016 7:49 am

Was browsing Avherald recently and came across an incident where a Lufthansa A340 performed a high speed rejected takeoff due to an out out of limit sudden tailwind. My question is how pilots are notified that the tailwind has suddenly gone out of limits? Do they base it off of unusual airspeed fluctuations during takeoff or is there something more complicated to it? And wouldn't it put the aircraft at risk of an overshoot? Here is the article I'm referring to.
 
benbeny
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 9:26 am

IIRC modern aircraft like airbus has wind direction speed indicator on the navigational display...
besides, suddenly changing tailwind will drop your airspeed considerably, which is very likely to gain attention from the pilots...
I don't know about the procedure, but maybe if they're below V1 anyway, rejection is the best course of action
 
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Starlionblue
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 9:32 am

The ground speed is always displayed on the navigation display. On the 330/340 it is derived from a combination of inertial reference and GPS. Once you get above the speed at which the pitots work, you also have indicated airspeed. By combining these, the avionics display wind speed and direction on the navigation display.

The pilots would have to observe the wind speed and direction from this indication.

Without wanting to second guess the pilots too much, I'd say the situation would have to cause very serious concern for them to reject at 130 knots. Speculating, perhaps they were rather close to performance limits for this particular take-off, and thus there was not much margin in case of an engine failure.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
greg85
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 10:38 am

Pilots are not notified if the wind goes out of limits, and air traffic control wouldn't know what those limits are. A wind vector is displayed above 100 knots. Rejecting takeoff for this reason is not within the standard Airbus FCOM reasons to stop, unless they were in a situation where they clearly could not takeoff. It's easy to judge while sitting here at 0 knots. But if they were worried about there takeoff performance calculations being invalid and not garunteeing reaching screen height by the end of the runway, they should have been equally worried about their accelerate stop distance being affected. But it's hard to know all the facts. How close were they to their calculated V1? Were they already at TO/GA power? How much did the wind change?
 
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Starlionblue
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 10:41 am

Greg85 makes some excellent points.

One wind related reason to reject would be getting a windshear alert.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
benbeny
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 10:56 am

A Lufthansa Airbus A340-300, registration D-AIGO performing flight LH-543 ..., was accelerating for takeoff from Bogota's runway 13R when the crew rejected takeoff at high speed (about 130 knots over ground) reporting a gust caused a tailwind component beyond limit.
....

Forgive my ignorance, but doesn't that imply a windshear? Surely the active runway will not have tailwind component?
 
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Starlionblue
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 11:46 am

benbeny wrote:
A Lufthansa Airbus A340-300, registration D-AIGO performing flight LH-543 ..., was accelerating for takeoff from Bogota's runway 13R when the crew rejected takeoff at high speed (about 130 knots over ground) reporting a gust caused a tailwind component beyond limit.
....

Forgive my ignorance, but doesn't that imply a windshear? Surely the active runway will not have tailwind component?


Tailwind component on takeoff is not uncommon. Airliners have no problem with some tailwind, typically less than around 10 knots.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
benbeny
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 2:16 pm

Starlionblue wrote:
benbeny wrote:
A Lufthansa Airbus A340-300, registration D-AIGO performing flight LH-543 ..., was accelerating for takeoff from Bogota's runway 13R when the crew rejected takeoff at high speed (about 130 knots over ground) reporting a gust caused a tailwind component beyond limit.
....

Forgive my ignorance, but doesn't that imply a windshear? Surely the active runway will not have tailwind component?


Tailwind component on takeoff is not uncommon. Airliners have no problem with some tailwind, typically less than around 10 knots.

Thanks! Nice to learn something new everyday :)
 
BoeingGuy
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 4:42 pm

Boeing airplanes are allowed to takeoff with a 10 knot tailwind as baseline. There is an option to allow 15 knots, which is only a paperwork change.

At first, I thought the A340 pilots error-ed badly by doing a high speed rejected takeoff that doesn't meet one of the normal criteria - engine failure, fire, etc. Then as I think about it, they may have observed the un-annunciated windshear criteria. At least in the Boeing manual there is a list of criteria that is considered to be a possible windshear, even if the Predictive Windshear alert doesn't annunciate. A rapid change is airspeed is one of them.

I suspect that Avhearld didn't tell the whole accurate story. I highly doubt the pilots did a high speed RTO solely because the tailwind component increased slightly above the limit.
 
BravoOne
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 4:51 pm

As Boeing Guy has said, you can buy the additional 5kts for a price. Operators that have this need, had this added to their Manuals. An example would be CAL in the 757 would arrive at a particular Central American early in the morning when the traditional winds were in excess of 10kts on the ILS runway which they needed due to low ceilings. By opting for the additional 5 kts they were able to land thus saving them thousands in irregular ops. Not all operators have this issue and have not purchased the paperwork to make it happen.
 
rcair1
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 6:01 pm

BravoOne wrote:
As Boeing Guy has said, you can buy the additional 5kts for a price. Operators that have this need, had this added to their Manuals. An example would be CAL in the 757 would arrive at a particular Central American early in the morning when the traditional winds were in excess of 10kts on the ILS runway which they needed due to low ceilings. By opting for the additional 5 kts they were able to land thus saving them thousands in irregular ops. Not all operators have this issue and have not purchased the paperwork to make it happen.


What are they buying, if it is a paperwork change. That means the a/c without the paperwork have the same performance.
rcair1
 
BoeingGuy
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 6:57 pm

rcair1 wrote:
BravoOne wrote:
As Boeing Guy has said, you can buy the additional 5kts for a price. Operators that have this need, had this added to their Manuals. An example would be CAL in the 757 would arrive at a particular Central American early in the morning when the traditional winds were in excess of 10kts on the ILS runway which they needed due to low ceilings. By opting for the additional 5 kts they were able to land thus saving them thousands in irregular ops. Not all operators have this issue and have not purchased the paperwork to make it happen.


What are they buying, if it is a paperwork change. That means the a/c without the paperwork have the same performance.


There are a number of things that customers can pay extra for Manual Updates only. You are paying for the engineering, testing and certification efforts. Here are a few examples:

- Tailwind component, as noted
- Increase in certified landing altitude from 8400 feet to 9500 (or 9600, I forget). Anything higher does require an equipage change.
- Increase in Max Gross Weight.
 
OzzyPirate
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Re: Tailwinds during takeoff

Wed Nov 30, 2016 11:41 pm

As above. My operator has the 15 knot tailwind limitation for the 738. If a tailwind exists or is likely to exist (for instance, if the wind is reported as variable) that will be factored in the takeoff performance calculations. Unlike a headwind component where we can accept a reduction in headwind (within strict limits) without recalculating performance, a single knot increase in tailwind requires a recalculation.
 
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Horstroad
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Re: Tailwinds during takeoff

Thu Dec 01, 2016 12:29 am

How is wind speed taken into account when V speeds are calculated, especially V1? With 10kts tail wind you have a much higher actual ground speed than with 15kts head wind at the same indicated V1 which means longer braking distance, more kinetic energy to be absorbed by the brakes etc. Will you have a lower V1 when taking off with tail wind?
 
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Starlionblue
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Re: Tailwinds during takeoff

Thu Dec 01, 2016 12:55 am

Horstroad wrote:
How is wind speed taken into account when V speeds are calculated, especially V1? With 10kts tail wind you have a much higher actual ground speed than with 15kts head wind at the same indicated V1 which means longer braking distance, more kinetic energy to be absorbed by the brakes etc. Will you have a lower V1 when taking off with tail wind?


A tailwind would indeed decrease V1, but only if the runway is limiting enough. If the runway is long and dry, most likely V1 is limited by Vr because you'd rotate before the point at which braking becomes an issue.

A wet runway will have a much bigger effect on V1.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Florianopolis
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Re: Tailwinds during takeoff

Thu Dec 01, 2016 5:56 am

Starlionblue wrote:
A tailwind would indeed decrease V1, but only if the runway is limiting enough. If the runway is long and dry, most likely V1 is limited by Vr because you'd rotate before the point at which braking becomes an issue.

A wet runway will have a much bigger effect on V1.


Is there a Vr fudge for gusty takeoffs?
 
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Starlionblue
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Re: Tailwinds during takeoff

Thu Dec 01, 2016 6:12 am

Florianopolis wrote:
Starlionblue wrote:
A tailwind would indeed decrease V1, but only if the runway is limiting enough. If the runway is long and dry, most likely V1 is limited by Vr because you'd rotate before the point at which braking becomes an issue.

A wet runway will have a much bigger effect on V1.


Is there a Vr fudge for gusty takeoffs?


We can only input steady winds for the RTOW calculation and we are typically rather conservative. Figure out the worst case gust scenario given the reported winds, and use that as an input.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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