After takeoff procedure

[tripleseven]

HI everyone....I was watching the movie Home Alone and I noticed when they show the AA aircraft departing there is a short puff of smoke coming from both wheels. Also in this picture I saw in this aviation book, I noticed the same occurance. I was just wondering what is going on? Im thinking the pilot may be hitting the brakes before retracting. Can anyone clear this up. Thanks.

[I_Money]

Yes, before retracting you want to make sure your gear is not spinning, and the easiest way to do that is to use the brakes.

[aloft]

Heck, I sometimes do that in a 172 if the wheels (particularly the nose gear) start shimmying after takeoff.

[I_Money]

Aloft you raise an interesting point. Any Cessna guy has experienced a rattle after take off, and we presume it is a form of shimmey, and apply brakes, however the 172 does not have a nose wheel braking system, however by applying the brakes it solves the problem. Can anyone explain what is going on.

[A300Capt]

Brake pressure is automatically (usually through the alternate brake system) applied to the main gear wheels when the gear handle is raised.

With the main gear wheels, gyrosocpic forces are 90 degrees to the direction of motion to the wheels and could cause problems if a wheel is having rotational problems or out of balance. This is different for the nose gear which don't have the same gyrosocpic forces working against it as the rotational forces are in the same direction of the gear movement.

The gear bay is also a tight area and you really don't want a tires spinning at 120+ mph in that closed area. Generally, main hydraulic reservoirs are also located in the wheel wells.

[eodfe]

On a P-3 they were called inflight brakes. If I remember right, after the gear handle is placed in the up position, the brakes are automatically and slowly applied so the wheel will stop rotating prior to it entering the wheel well. This system is only on the main gear since the nose gear did not have brakes. We knew that the inflight brakes system wasn't working when we were doing touch and goes. Part of the landing checklist is to check the brakes and monitor the hydraulic gauges for a fluctuation, when you heard a thump, you knew one of the main gear or possibly both were still spinning.

[aloft]

Oh man! And I hadn't even been drinking when I wrote that! Of course there's no brake on the 172's nose gear....duh! That's pretty funny.

I think I probably made that association because the sensation feels similar to nosewheel shimmy just after touchdown. In thinking about it more clearly (hopefully), I'm sure it's a result of gyroscopic forces on the still-spinning main gear.

Nose gear...sheesh!

[El_Cid_Av8or]

After takeoff it is a very good idea to move your toes up on the rudder pedals and press the brakes to stop the free-spinning main gear wheels. This will save the bearings since the wobble from the gyroscopic forces will not be a factor. Yes, the gyroscopic forces exist when a fixed-gear GA single lifts off though not to the extent of one which has retractable gear (when the gear is coming up). This is not as bad in low-wing planes as in high-wing planes since the low-wingers don't have the gear leg length that the high-wingers have. Think of the spring gear leg length on a Cessna 172 compared to a Piper Warrior. When the weight comes off the spring leg, the gear leg extends downward - this is where the gyroscopic forces come into play.

Also, make sure you DON'T have your toes on the brake section of the rudder pedals while you're on the takeoff roll and also on the inital landing roll. There have been cases where pilots have ground-looped a plane because they applied rudder correction for cross-wind on the runway and pressed the brake as well. Not good, Mav.