747 Landing Gear

[Sibcy999]

This may be the wrong place to try to get this answer so please direct me elsewhere if appropriate. I have a friend who swears that the main landing gear on a 747 can articulate left and/or right of center when landing in case the plane has to dramatically crab when landing so the main landing gear can face straight down the runway. I told he was mistaken and that the gear could not be controlled as such. The correct answer would be appreciated.

The is same friend and also said that is a 747 lost all power it could move along the surface of the ground a distance of 7 times the altitude of the plane. I have not idea if this is anywhere near correct. It seems it would depend a lot on the airspeed at the time, but maybe not. Any help with this aspect of the 747 would be appreciated. (A beer rests on the answer to the first question:O)

Many thanks

[Cornish Jack]

747 gear - NO!
737 - possibly

[grounded27]

Not in flight but on the ground the body gear assists in steering allowing for tighter turns.

[spekesoftly]

B747 inboard main gear is steerable, but only for ground manoeuvring, not to aid cross-wind landings. (I believe the C5 Galaxy did originally have such a system for use during landing).

I would think a 747 would glide about 12 times its altitude - approx 70nm from 35,000ft.

Here's an example: Welcome to Eric Moody (http://www.ericmoody.com/)

[Dan Winterland]

The body gear on the 747 roates up to 7 degrees (IIRC - 8 years since I've flown one) to assist steering. On the classic it is manually activated by a switch which is used once lined up on the runway. On the -400, it's automaticly deactivated once above 20 knots groundspeed. The 737 is often seen crabbing slightly on the ground, but this is due to it's rather loose anti-torque link system and not by design. The 777 had a three axle main bogie and the last axle is steerable in much the same way. The 747 does have an impressive turning circle as a result. I used to fly for an airline which had 747s and A340s. The 747s were used to fly to the smaller caribbean destinations for partly the reason the 747 was more manoeuvreable on the ground.

Interestigly, on wet or slippery runways, Boeing reccomment that the 747's are laded with the drift on. This is beacuse the body gear touches down first and being behind the centre of pressure, slews the fuselage in line with the direction of travel. I've done it a few times (once or twice unintentionally!) and it works, depsite feeling uncomfortable.

The C5A Galaxy did have the swivelling gear for crosswind landings, but it was expensive to maintain and has been deleted in the C5B model. Originally, it had to be set manually based on the reported cross wind and I gather it led to some interesting moments when the reported met wind was either wrong or varying down the runway. A later modification had an input from the IRUs so that it was correct on touchdown. However, the B52 retains a swivelling gear system and has to because of the design.

[spekesoftly]

On the classic it is manually activated by a switch which is used once lined up on the runway.

Dan,
Should that read deactivated? Thanks

[grounded27]

The 747 does have an impressive turning circle as a result. I used to fly for an airline which had 747s and A340s. The 747s were used to fly to the smaller caribbean destinations for partly the reason the 747 was more manoeuvreable on the ground.


First commercial aircraft I taxied (dual tiller from the rt seat), Those outboard engines are great in a tight turn as well to help swing the nose around.

[Desert185]

Question #1: No. Body gear swivels on ground only for steering during taxi.

Question #2: Gliding distance depends on the weight. Longer distance for heavier weights and shorter distance for lighter weights. There is no performance chart for this. At medium weights, three times the altitude to be lost is generally used for descent planning distance, i.e. 35,000' to lose...begin descent 105nm out (no wind).

[galaxy flyer]

For max range glide, the achievable distance remains the same--weight just changes the IAS that results in optimum AoA.

[Dan Winterland]

''Should that read deactivated? Thanks''

Of course!

''For max range glide, the achievable distance remains the same''

Agreed. It's the lift/drag ratio that determines the glide angle and therefore range, and this ratio is the same regardless of weight. Proof of this is that many gliders (sailplanes) have water ballast tanks to increse their wing loading and gliding speed. This is done on days where there is strong lift and the faster speed gliding beteen thermals is a better tradeoff for the slightly increased sinking speed. The ballast can be dumped for when the lift reduces later in the day, or for landing as gliders are rarely cleared for landing with ballast on board.

[Desert185]

Joe le Taxi Correct, galaxy flyer. L/D does not vary with weight. D185's is a common misconception.

Also his 3x altitude distance assumes just above idle power. Deadstick, you can work on roughly 2-2.3 x alt. Although of course, if you start at Vmo, and finish the glide at Va, then the glide distance will be extended.


I never said anything about L/D. In a 747 descent, landing weight can vary from 400K to 630K, depending on the model. At the heavier weights, descent rate will be less than lighter weights, given the same IAS profile.

As I said, at medium weights, 3X the altitude to lose works very well as a descent point. If lighter or heavier/headwind or tailwind, there is a distance adjustment. All this assumes an idle power descent.