OneX123
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Delayed Gear-up out of KBOS

Sun Dec 31, 2017 6:21 pm

I recently flew a Norwegian 787 out of Boston. Right before takeoff, the pilot advised that due to procedure the gear would stay extended for about 2 mins after takeoff. I knew this was common during hot weather, however, the flight departed at 9pm and the temp was 0 degrees F. Any suggestions what the reasoning for this would be?
 
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DocLightning
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 6:30 pm

During a long taxi, frequent use of the brakes can cause them to get a little on the warm side. Not so warm that they are a danger if a takeoff needs to be aborted, but warm enough that you wouldn't want to pull them into the gear bay without cooling them off first. So they just let them dangle for a few minutes, they cool down, and then they retract.

Also, depending on how short the turn was, they can also be hot from landing.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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OneX123
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 6:32 pm

DocLightning wrote:
During a long taxi, frequent use of the brakes can cause them to get a little on the warm side. Not so warm that they are a danger if a takeoff needs to be aborted, but warm enough that you wouldn't want to pull them into the gear bay without cooling them off first. So they just let them dangle for a few minutes, they cool down, and then they retract.

Also, depending on how short the turn was, they can also be hot from landing.


Appreciate the response! It was a quicker turn for a wide body and definitely a longer taxi. Thought the break temp was only an issue in warmer weather but good to know. Thanks again!
 
7673mech
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 6:35 pm

It is also a procedure when 1 brake is inactive.
 
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zeke
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 6:37 pm

A brake inop.

Normally after liftoff when gear up is selected up brakes are applied to stop the main wheels from rotating. The rotating wheel assemblies are like gyros, if the gear is retracted with them rotating it causes torque and vibration tp be ransmitted to the Airframe.

Leaving them down for 2 minutes allows the wheels with the inop brakes to run down to reduce the toque and vibration when retracted. If there was an engine failure they would retract the gear regardless.

To understand this more look at the spinning bicycles wheel torque on the internet, bound to be some videos.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
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DocLightning
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 7:06 pm

zeke wrote:
A brake inop.

Normally after liftoff when gear up is selected up brakes are applied to stop the main wheels from rotating. The rotating wheel assemblies are like gyros, if the gear is retracted with them rotating it causes torque and vibration tp be ransmitted to the Airframe.

Leaving them down for 2 minutes allows the wheels with the inop brakes to run down to reduce the toque and vibration when retracted. If there was an engine failure they would retract the gear regardless.

To understand this more look at the spinning bicycles wheel torque on the internet, bound to be some videos.


Oh, that, too. And also, pulling a spinning wheel into a wheel well is just asking for trouble. Best not.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan
 
GalaxyFlyer
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 8:23 pm

Ice or slush on the surfaces are also a reason-the air flow removes it preventing iced brakes or gear locks.

GF
 
mmo
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Re: Delayed Gear-up out of KBOS

Sun Dec 31, 2017 9:09 pm

As GalaxyFlyer wrote, in the winter, the most likely reason for delaying gear retraction would be contamination. Leaving the gear down allows the airflow to get the slush and other debris which has built up during the taxi to be removed via the slipstream. The probability of hot brakes due to a short turn or heat build up during the taxi is extremely low in the wintertime.
If we weren't all crazy we'd all go insane!
 
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zeke
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Re: Delayed Gear-up out of KBOS

Mon Jan 01, 2018 1:19 pm

The key reference is “two minutes” that is for MEL 32-45-01 for a brake inop. You don’t need two minutes to blow off slush

MEL reference http://fsims.faa.gov/wdocs/mmel/b-787_rev%2012.pdf
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
GalaxyFlyer
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Re: Delayed Gear-up out of KBOS

Mon Jan 01, 2018 3:16 pm

Zeke,

Curiously, I was struck by the time of 2 minutes, which is the same time in the C-5 when the retract brake system was inoperative. Funny, that.

GF
 
WIederling
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Re: Delayed Gear-up out of KBOS

Mon Jan 01, 2018 6:38 pm

GalaxyFlyer wrote:
Zeke,

Curiously, I was struck by the time of 2 minutes, which is the same time in the C-5 when the retract brake system was inoperative. Funny, that.

GF


Which is probably due some long time ago determined interval that gets applied when this comes up for regulation?
Murphy is an optimist

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