AJ From Australia, joined Nov 1999, 2380 posts, RR: 26 Reply 1, posted (10 years 4 months 3 days 17 hours ago) and read 2487 times:
During the retraction cycle of the undercarriage the rotation of the wheels is stopped to prevent gyroscopic rotational forces. When the brakes are applied it can cause any excess grease/dirt etc. to burn off as the brakes are absorbing quite a bit of energy, iplus the heat absorbed during taxi.
You can imagine the stress the brakes would be under in a rejected takeoff!
AA737-823 From United States of America, joined Mar 2000, 5547 posts, RR: 11 Reply 3, posted (10 years 4 months 3 days 17 hours ago) and read 2432 times:
Actually, my understanding is that, while both of the above explanations are correct, the actual stuff you see is carbon- because they're carbon brakes, not steel anymore. When the system 'steps' on the brake pedal during retraction, the wheels stop pretty quickly, producing a puff of black carbon powder from the braking surfaces.
Jwenting From Netherlands, joined Apr 2001, 10213 posts, RR: 20 Reply 6, posted (10 years 4 months 2 days 18 hours ago) and read 2190 times:
It only happens when a lot of brake power is used during taxi or when putting on the brakes before gear retraction.
It might also have to do with environmental conditions (if there's a lot of wind the stuff would probably disperse quickly so you don't see it for example).
Bio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7 Reply 9, posted (10 years 4 months 2 days 7 hours ago) and read 1963 times:
AJ: I'm probably not very familiar with the subject, but I always thought that the stopping of the rotating wheels was to prevent further damage given a tire blowout during retraction inside the bay. Is that correct?