NJTurnpike
Topic Author
Posts: 565
Joined: Mon May 01, 2000 8:24 pm

Runway Alignment

Wed Jan 03, 2001 10:04 pm

Can anybody answer a question[s] which has had me stumped for months? How are the wheels of an airliner aligned (or set straight) for takeoff and landing? In all the times I have been staring out of window seats on takeoff, the plane always head "straight down the middle" of the runway, never seeming to veer to either verge.

Is the undercarriage locked before thrust is applied, or is there some degree of nosewheel steering that goes on until rotation? Landings also seem uncannily straight, especially on 747s with full wheel brakes, spoilers and thrust reverse applied, which might otherwise (in my mind anyway) ensure the aircraft would end up in a ditch to the side of the runway. The precision of such landings never fails to impress me.

Thanks for any answers to my ramblings,

~NJTurnpike
 
Airbus_A340
Posts: 1439
Joined: Sat Mar 11, 2000 8:41 pm

RE: Runway Alignment

Wed Jan 03, 2001 10:16 pm

I was on a Cathay Pacific A340-313X 2 weeks ago coming from Malaysia. Thrus was applied and we went down the runway, i could have told u that if u were standing on the end of it, u would have seen the plane all over the place, the passengers were being thrown left and right, cause the pilot kept tryng to line the plane up during takeoff roll.

I know that on the Airbus, the tiller is used for the tight turning, and going down the runway when it reaches a certain speed the rudder peddles controling rudder and nosewheel becomes effective because of the speed of the airflow.

Landing on the A340 was great, (touchdown)!......but again we were still all over the runway!

I went to the cockpit afgter the flight and was told that the pilot was a trainee and there was also an observer.

Kind Regards
Trevor AKA Airbus_A340
People. They make an airline. www.cathaypacific.com
 
Guest

RE: Runway Alignment

Thu Jan 04, 2001 2:42 am

Quick and dirty...

Taxiing the aircraft, a steering tiller is used to make turns which is normally located on the Capt's side (some airlines have a tiller on the Capt's and F/O's side). The rudder pedals have a hydraulic/mechanical connection to the nose wheel(s) and can be used to steer straight ahead and make very limited turns.

Once on the runway and the aircraft is aligned with the centerline for takeoff, the rudder pedals become the main steering input. During the early part of the takeoff roll steering comes from the hyd/mechanical connection from rudder pedals to nose gear. After about 40kts of groundspeed the rudder on the tail begins to help through aerodynamic forces. It much like a rudder of a boat becoming more effective the faster you go. This transition from hyd/mech to aerodynamic steering is basically invisible and progressionary to the pilot as he is just manipulating the rudder pedals the whole time.

On landing, the aircraft is allowed to naturally crab (weathervane) into the wind while on final approach. Just prior to touchdown rudder pressure is applied to align the longitudinal axis (imaginary line from the nose to the tail) of the aircraft with the centerline of the runway. In otherwords, you want the body of the airplane in alignment with, and going straight down the runway as much as possible.

The B-52 is the only airplane I know of that has the capability to mechanically swivel their main gear for x-wind landings. This allows the airplane to remain in a crabbed attitude while the landing gear is aligned with the runway centerline.

Landing in a crab (body of the airplane is not aligned with the centerline) is acceptable and even recommended during slippery runway conditions. As speed slows the rudder becomes less aerodynamically effective and steering is accomplished through the hyd/mech/rudder connection as I talked about in the beginning. The Capt finally transitions to using the tiller to taxi off the runway to the gate.

Nothing to it! 
 
modesto2
Posts: 2681
Joined: Sun Jul 16, 2000 3:44 am

RE: Runway Alignment

Thu Jan 04, 2001 2:57 am

I landed aboard a UA 777. Immediately after touchdown, I could feel the aircraft moving left and right of the runway centerline. It was obvious that the plane was not centered and the pilots were using the rudder to adjust the alignment. This goes to show that it's not always perfect!
 
wyorca
Posts: 29
Joined: Fri Aug 18, 2000 3:31 pm

RE: HeavyJet

Thu Jan 04, 2001 1:00 pm

When using the "kick out" method to correct for a cross wind in a large transport a/c, how much aileron input is needed to prevent the a/c from drifting with the crosswind? Or is the forward momentum of the aircraft enough to keep the aircraft from drifting in the last few feet before reaching the ground?

Thanks

S Kasakow
 
NJTurnpike
Topic Author
Posts: 565
Joined: Mon May 01, 2000 8:24 pm

RE: Runway Alignment

Thu Jan 04, 2001 10:49 pm

Thanks for the responses (particularly HeavyJet's explanation). Guess I've been particularly lucky with well centered takeoffs and landings, with only one 'cabin-throwing' incident - a USAir DC9 landing at PHL last year - in my 16 years as a passenger.

That said, I would have loved to have been present for this landing.


Click for large version
Click here for full size photo!

Photo © Alan Tsui



~NJTurnpike
 
Guest

RE: Runway Alignment

Fri Jan 05, 2001 12:44 am

>>When using the "kick out" method to correct for...<<

Wyorca,

That depends. Strong x-winds require aileron input proportional to the amout of rudder being used and can vary from moment to moment. I've landed with quite a bit of control wheel deflection to counteract rudder input and drift.

After about 10 degree deflection of the control wheel the spoilers begin to help. Surprisingly to many, the big jets such as the B75/76 and A300, not to mention many others, are very maneuverable and handle x-wind conditions pretty well. The A300 has a max demonstrated x-wind component of 32 kts.

If a pilot lands in a crab he'll try to get the body of the aircraft lined up with centerline before lowering the nose gear. After nose gear touchdown, getting your feet to the top of the rudder pedals in order to apply brakes can be tricky. Autobrakes are a big help in x-wind landings and I use them everytime. This way you can keep the balls of your feet on the bottom of the rudder petals and just concentrate on steering while the autobrakes initially do all the braking.

Usually when you feel the aircraft moving from side to side after landing it was the pilot trying to move his feet up to the brakes. I know, I've been there! 
 
Red Panda
Posts: 1433
Joined: Wed Jun 14, 2000 12:58 pm

RE: Runway Alignment

Fri Jan 05, 2001 2:18 am

I think the leading wheels would automatically go straight during takeoff. Just imagine when you are driving a car down on the freeway, you will get the front wheel straight even you don't touch steering wheel. If this doesn't happen, then you better check the alignment of the wheels. Also, imagine how to steering wheel goes back to its starting position after making a turn. In the middle of a turn, you let go the steering wheel and the wheel goes back to its original position. I think a/c does the same too. On the other hand, you still have to sort of hold on the steering wheel a bit when driving straight in order not to let the car out of course. Hope this analogy helps.


Regards,
R panda
 
wyorca
Posts: 29
Joined: Fri Aug 18, 2000 3:31 pm

RE: Runway Alignment

Fri Jan 05, 2001 10:14 am

Thanks HeavyJet,

I use the "side-slip" method during x-wind landings but then again, I don't fly large transport aircraft. I have noticed that about everything larger than ATR's tend to use the "kick out" method verses the slip. Why is this? It would seem to me that heavy airliners would be very suseptable to damage when they are side loaded?

Thanks
 
Guest

RE: Runway Alignment

Fri Jan 05, 2001 3:29 pm

Wyorca,

A few reasons...

One, it's easier to allow the airplane to naturally crab into the wind down final. Smaller corrections need to be made using this technique (IMHO) in a bigger transport aircraft. It's mass, speed, inertia and higher wing loading make it more stable during gusty or x-wind landings.

Second, using the sideslip method on final tends to block smooth airflow on the downwind jet engine(s) and may result in compressor stalling. This results from the fuselage blocking airflow (more noticable with aft fuselage mounted engines ie..727, DC9 etc)

Third, unnecessary prolong stress on the vertical stabilizer and rudder trying to hold the sideslip. Remember, the airplane wants to naturally crab into the wind...so why not let it.

Lastly, due to the length of the fuselage in large jets, it makes the passengers very uncomfortable when a pilot sideslips on final (not that the boxes I carry now care but back in my commuter flying days... )

The sideslip method works well for small aircraft even though I still prefer the kickout method. However, the "kickout" method works best for larger aircraft.

Some bigger jets will land in a crab, such as a DC8 to prevent scraping an outboard engine pod using the kickout method. Those big CFM's don't have alot of ground clearance.
 
wyorca
Posts: 29
Joined: Fri Aug 18, 2000 3:31 pm

RE: Runway Alignment

Sat Jan 06, 2001 1:40 pm

Thanks a million 

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