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shon7
20th Mar 2002, 21:52
In a 747 landing or any widebody for that matter, how do pilots know when the wheels have touched the ground. Can you feel it as you do smaller aircraft. I asked a 747 F/O this question and he said that on contact the speed brake automatically retracts. However, I was watching one of those airliner videos and it was an MD-11 and the brake retracted well after the nose wheel touched the ground- (it was an autoland so I don't know if this accounted for the speedbrake action).

18-Wheeler
20th Mar 2002, 22:45
You land them just like a 'little' plane, ie, you look out the window at the far end of the runway and at the right height above the ground pitch up to the right flare attitude.. .It's a touch more complex than that though, especially when you first start flying them!. .There's a couple of techniques that you can use, and the Qantas one is to set the radar altimiter to 50ft, and when it goes 'DING' then you count "one bananas, two bananas", then flare - No kidding, that's what they taught me. <img border="0" title="" alt="[Wink]" src="wink.gif" /> . .The other way is to simply flare at 30ft, and let the plane settle by itself onto the runway, and this is close to what the autopilot does when it lands the plane.. .After a while though, you don't need the radar altimeter or the Flight Engineer calling the heights for you, you can just pick the right height by eyesight. Takes a while though, as the plane is so large.. .I also flare the beast at different heights and slightly different rates with different weights. The lighter you are, say, a featherweight 190 tonnes, you can get down to a little less than 20ft no probs and hardly pitch up at all to make a good landing. Up around 280 tonnes, you have to leave the power on as you pitch the nose up at about 30ft, then pull the power back as the plane settles.. .Other people have different techniques to mine that no doubt also work well.

18-Wheeler
20th Mar 2002, 22:56
Okay, after a quick proof read I think I might try to actually answer your question! <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .You know you're on the ground because the ground safety relay kicks the solenoid in the landing gear handle, so that you can't select gear-up on the ground. You also get the auto-spoilers moving the spoiler lever all the way back to the fully deployed ground spoiler position, and that makes a quite noticeable noise.. .There's also a LOT of other little things that happen that tells the plane that it's on the ground, but those two above really tell the pilot that the main wheels are on the ground.. .It takes a while to get a good landing technique in the thing, because you can actually land the plane too softly - The landing gear on the 747 sits at quite an angle with the wheels off the ground, and so when you touch down the un-tilting of the wheel trucks tells the logic circuitry that the plane is on the ground.. .But!. .You don't actually have to un-tilt the trucks much at all, so it's possible to just get them touching the runway, the plane says 'Okay, I'm on the ground so I'll throw the spoilers up all the way', and when that happens a fair chunk of the wing stops making lift, and so the plane drops a few feet onto the runway.. .Hence the phrase occasionally heard on the flight deck, "Well, the first one was nice!" <img border="0" title="" alt="[Smile]" src="smile.gif" /> . .On a landing with the sink rate too high, you naturally feel the plane 'thump' onto the ground. Boeing have kindly built the 747 so you can land it at max landing weight (~285 tonnes) and not flare it at all, and it will suffer no damage.. .On a good landing, the sink rate is such that you can hardly feel the plane touch down at all, and the only feeling you get is as the wheels spin up.

start sequence 312
20th Mar 2002, 23:24
shon7, Let me try.. .MD11 pilots can expect or feel the touchdown based on these procedures, during manual or auto landings: . .First, the GPWS provides radio altitude callouts "50", "40", "30", "20", "10" feet during landing (and approach as well, but this is not the case).. .Second, normally they feel the touchdown even on wet runways and if they don't make a correct flare they'll feel even more <img border="0" title="" alt="[Smile]" src="smile.gif" /> (sorry!). .Third, if prior to landing the speedbrake handle is in the armed position and the flaps 31 degrees or more After main wheel spinup the spoiler handle will move automatically (if not, it must be manually deployed) to a partial extended position (there are 10 spoiler panels, 5 on each wing) and then after nose gear touchdown the handle will move to the fully extended (I suppose this handle movements you watched on that video). This automatic procedure is used to avoid a tailstrike because MD11 has a pitch-up tendency after spoiler extension.. .Rgds.

shon7
21st Mar 2002, 02:30
Thanks for the detailed replies guys. To continue on that question I had a few queries about reverse thrusters/ thrust reversers.. .Once and for all could you tell me if 747s have Reverse Thrusters or Thrust Reversers. . .. .Also I was under the assumption that thrust reverse is only applied after nose wheel contact with the ground. However, in some pictures I saw recently on the web, it seems like thrust reverse has been applied immediately after main gear touchdown. So how does this work?

Hand Solo
21st Mar 2002, 03:53
Not sure what a Reverse Thruster is? In my airline they just call them thrust reversers. I don't fly the 747 but having watched lots of them land close by most deploy the thrust reversers immediately after main gear touchdown and long before nose gear touchdown.

jetboy
21st Mar 2002, 05:52
Shon7,there are lots of good books at the library on big jets,and I suggest you start there.Lots of good sites on the 'Net as well. Firstly, pilots (and passengers)can feel the touchdown,(it happens at about 2 ft/sec). Double bogied gear (4 wheels)contact in two phases (usually rear followed by front). The pilot must then de-rotate to bring the nose wheel onto the runway (this can also be felt by all on board, depending upon pilot skill). Various sensors on the landing gear determine 'weight on wheels' or ground mode. Some equipment, like thrust reversers or ground spoilers, require main gear on ground. Wheel speed (rotating) can also be used on some aircraft to initiate spoilers and autobrake. What you saw in the photo, was reverser deployment, with the nose gear still in the air. This is possible, although the interlock on the reverse thrust levers would not enable the pilot to 'pull reverse thrust' (increase engine power) until the nose gear was compressed(otherwise,the nose may slam down onto the runway). Thrust reversers (not reverse thrusters)merely block the fan bypass air (the air that is produced by the fan, but does not flow through the engine core)and reverse its direction through cascade vanes. The fan produces about 70% of an engine's thrust, so by reversing this air, braking can be achieved. Low or no bypass engines use reversing buckets behind the exhaust. These were tried on high bypass engines in the early days, but the cost, weight and maintenance didn't make them feasible. Reversing the fan air, along with wheel braking and ground spoilers does the job just fine.

18-Wheeler
21st Mar 2002, 06:21
On the 747's, you can pull reverse thrust as soon as the mains are on the ground, so it's normal to have full reverse thrust going before the nosewheel touches down.. .Whilst the brakes can certainly stop the plane very quickly indeed, you often have to give thought to the next sector that the plane is doing, and so it's not uncommon to not use the autobrakes, as they will heat the brakes up quite quickly even if you get reverse in as fast as you can.. .Full reverse thrust in the Classic is about equal to minimum autobrake, and so if you land like that you can simply use reverse down to about 70kts before even touching the brakes at all, thus they stay a LOT cooler.. .You'd think of doing this in places like Saudi Arabia, where the ambient temperature is usually very high and so there's not a lot of brake cooling. (And you don't want ANY hiccups to stop you from leaving!! <img border="0" title="" alt="[Smile]" src="smile.gif" /> <img border="0" title="" alt="[Smile]" src="smile.gif" /> )