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dmwalker

Leaving Landing Gear Down After Takeoff

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When a B787 was given clearance to take off at our local airport recently, the response was "Clear takeoff 24R and be advised we have to leave our gear down for two minutes". What was the purpose of that? I assume it was some sort of routine test.

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Turnaround time was about three and a half hours and from gate to runway took only 10 minutes so I thought it couldn't be that. In any case, the recommendation for brakes overheating during takeoff is to keep the landing gear extended for 12 minutes.

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In the MEL for a lot of 747 variants, if there is a failed brake but that failure still allows departure of the aircraft, it is recommended that you should wait two minutes after lift off until retracting the gear. There are a couple of reasons for this...

In the first place, many sophisticated airliners automatically brake the wheels when you select 'gear up' with the landing gear lever, this is primarily because the gyroscopic precession forces from a big rotating wheel can put sideways stresses on the retraction mechanism, so wheel rotation is stopped upon retraction in order to prevent that from occurring, but of course it wouldn't happen if there was a failed brake, so waiting two minutes would ensure the wheel had spun down after take off.

The other reason is that if there is any dirt or stones on the wheels jammed in the tread, it can be thrown up into the gear wheel well if a spinning wheel is stopped suddenly upon final retraction when it touches the rubber strips which are often present in in the gear bay that are there to stop the wheels rotating in the bay. Anything flung off a spinning tire which suddenly stops in the landing gear bay could potentially damage systems located in the wheel well, typically hydraulic and electrical lines, but also things such as lights mounted on the landing gear.

This is why pilots usually get taught to dab the brakes just after take off on things like Cessnas, Pipers, Mooneys and Beechcraft if they have retractable gear and depart from grass strips, since doing that throws any sh*t off the tires whilst they are still out in the airflow prior to the wheels coming up and muddying up the gear bay. As noted, It is not unknown for dirt and stones to get thrown off tires in the gear bay and then cause damage if that is not done, this is why 737s and 727s which were fitted with the rough field operation modification kit were also fitted with mesh screens in the landing gear bay as part of that kit, in order to prevent damage of that kind upon retraction of the wheels.

Of course it never does any harm to leave the gear down for a bit anyway after take off, since the brakes could be hot or the tires could be too if the aircraft had a particularly long take off roll on a hot day. There have been cases of airliners being lost not long after take off due to fires in the gear bays because of hot tires and brakes being retracted. Notably a DC-8 in Africa a few years ago and a 727 in Mexico some years earlier, although in both those incidents, the cause was traced to, in one case, an under-inflated main gear tire causing the other tires to take too much load and thus overheat and burst into flames, burning away the control hydraulic lines and in the other case, air being used to inflate a tire instead of nitrogen, which caused a nose tire to overheat then explode in the gear bay, severing the hydraulic lines to the control surfaces.

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