After a long sleep the topic returns to haunt us once more. For this truly is the Holly Grail of 737 questions.
It was first fought out on these hallowed forums in 1997 (under my previous username). Alas the link has long since been lost in the mists of time but some remnants of the original manuscript still survive to this day. They have been disidentified to protect the identity of the scribes.
Let the tale commence...
7/8/97: Can anyone tell me why it is that B737's crab along sideways on the
ground? Before anyone answers that the gear is not centered or whatever, I taxied
along behind one in SEL a couple of weeks ago pondering this point, when suddenly
the fellow decided he had had enough of crabbing to the left and began crabbing
the other way! I'm dumbfounded.
8/8/97: Re: 'Can anyone tell me why it is that B737's crab along sideways on the
ground?' I have noticed this. I assume that they all started off symetrical
when they left Boeings! Pondering the topic fills those wasted minutes in line
before departure and leads to numerous missed ATC instructions!
8/8/97: RE B737's WITH CRABS: 7/8/97. I am led to believe that the main gear on
B737's actually 'castor' a few degrees (I vaguely recall 7 degrees), presumably
for crosswinds, hence the "bent chassis" look.
8/8/97: Regarding 737's taxing in a "crabbed fashion" I flew 737's for 5 years
and I don't have any idea why they "appear" to be crabbing along!! While
following numerous 737's I have noticed it as well, maybe it's an optical
illusion or one of those parallax error things???
8/8/97: Reference the B737 crabbing - it's the torque reaction from the rotating
beacon that causes the crab - as to a sudden change in the crab direction, I can
only guess that the bulb failed at this specific instance or the skipper switched
the beacon off
8/8/97: Ref. Boeing 737 ground crab; I too have noticed this peculiarity when
taxiing behind 737s. I have forwarded the originator's question to Boeing at
their web site (
www.boeing.com). Hopefully, Boeing engineers will shed some light
on this subject. Until then, this is going to drive me nuts every time I see a
737 taxiing sideways. I'll post their response if and when I receive one.
10/8/97: Re: the B737 crabbing sideways while taxiing. I believe that it is a
design compromise introduced by Boeing to overcome the small angle of bank
allowed, during a crosswind landing, before striking the engines. The lateral
movement of the main gear will help to overcome the problem of having to land
with a crab angle. If you look closely at the B737 main gear you can see the cam
about which the gear swivels. The massive engineering problems that Boeing
encountered designing the B737 is interestingly documented in the book "Widebody"
(unsure of the author).
10/8/97: Re. Boeing 737/767 etc crab angle taxiing. The Steering geometry for
most aircraft right down to a PA28 rely soley on the scissor link pins to keep
the bogie/wheels straight. There is no tie rod as in the automotive equivilent.
So, a two thousanth of an inch clearance in the scissor link pins multiplys out
to be in exess of half an inch at the wheel axle line. Now hang this on a 100 ft
fuselage and the couple of thou. plus the distortion of tyre etc allows the
aircraft to crab. Any cross wind and or cross camber on the taxi way or runway
will augment the crab angle, as well as when one straightens up out of a turn the
residual angle can show its self. Go and have a look at the main gear strut and
check what keeps the bogie/wheels in line with the fuselage or running angle.
Note there is no pin inside the strut. The nose wheel suffers from the same
problem but it is attached to a steering ram and so it adjusts from the steering
input. Without giving a full sized tech drawing, even aircraft with bogie unlock
have the same malady.
10/8/97: Re B737 crab. The B737 has a very large fin. Where was the wind blowing
from? So depends the direction of crab. The poor little thing is always trying to
weathervane into wind. Any sailor knows that.
11/8/97: Well Gents, I am pleased that my question on the 737 crab has led to so
much discussion. I particularly like the theory on the rotating beacon. Next time
I go to work, I shall sit at the holding point and cycle the beacon on and off to
see if I can feel the torque kicking in and out on my aeroplane! If you have
counter rotating beacons (ie. the top one goes the opposite way to the bottom
one), is the effect cancelled out? Seriously, I shall be observing other types to
see if I can see this mysterious effect on them.
16/8/97: Ref; The Continuing 737 Ground Crab Saga; It's a little late, but here
is Boeing's response, if we can call it that, to our question; Why does the 737
crab while taxiing? Their Answer; "Dear Nigel, Thank you for visiting the Boeing
website. We hope you found it useful and informative. You asked for information
regarding the B737's crabbing capabilities. As you can imagine, we get thousands
of requests for information each month... blah, blah, too busy to answer each
query in detail...etc." I guess all we can surmise from Boeing's response is
that, as suggested in this forum by another contributor, the crab is indeed a
design "capability" and not a defect!
17/8/97: By Jove the 737 does Crab!
18/8/97: Re 737 crabbing. Another factor is tyre slip angle – tyres don’t always point
exactly the way there going due to flexibility in the sidewalls (ask a racing driver or
get someone to turn a parked steering wheel while you watch the tyre the tread stays
put but the wheel rim moves a few degrees). The 737 has a very big tailfin so any crosswind
will maybe affect this aircraft more than others. PS I thought that the reason that G & T’s
were not served on the ground was to stop the aircraft getting a torque reaction from too
much stiring, same as the anti-collision light theory. Get both of these acting together and
you could be on the grass before you know it!
19/8/97: The Boeing 737 crab is a manifestation of the coriolis effect. The motion is due
to the movement of the aircraft across the surface of the rotating earth. This is why the crab
occurs in the opposite direction in the southern hemisphere. If taxiing is attempted on the
equator, the aircraft will flip onto its back.
20/8/97: I can vouch with some authority that the dreaded Coriolis Effect IS the
prime culprit in the 737's tendency to crab. However, thankfully this is
counteracted to some degree in the Northern Hemisphere (except at very high polar
latitudes) thanks to the torque effect of the anticlockwise-rotation of the
anti-collision light in standard three phase 115V AC equipment as fitted to all
Boeing aircraft. Bitter experience has shown that the aircraft can become close
to uncontrollable below twenty knots if a polarity reversal is suffered after
pushback in the Main AC Bus, (which, surprising as it may seem, is NOT covered in
the MMEL - a grave oversight on Boeing's part). And just when you think you've
got all bases covered, to upset all one's most careful calculations completely,
the ubiquitous Phosgene Layer frequently comes into play, (particularly, I've
found, if there's a supervisory pilot on the flight deck at the time). Never
underestimate the ***** Phosgene Layer, chaps and chapesses. Let's face it. This
is a damned dangerous business. They just don't pay us enough, no sir-ee they
don't.
20/8/97: Re crabbing 737s. For those desperately bored individuals out there who
want to see every known theory on the subject, try
www.chicago.com/airliners/ and
search for the <Crooked 737s?> string. The site's fun anyway.
20/8/97: The B737 will NOT flip onto its back at the equator (19/8) due to
Coriolis. Boeing's C.U.N.T. (Coriolis Undercarriage Nosewheel Tiller) system
gives a bias to nosewheel steering while taxying, a system akin to something like
a Mach trimmer. This was a recent upgrade from C.R.A.P. (Coriolis Reducing APU
Philosophy) where the APU auto-vectored its exhaust to counteract Coriolis.
Trouble was the system was ineffective when the APU was U/S. In earlier days the
C.O.C.K. (Coriolis Over Centering Knack) was a pilot technique of overcentering
the nosewheel but the aircraft went T.I.T.S.U.P. (Taxying Into Tumble Spin Under
Power) if the knack was misused. Thus a C.O.C.K. caused a B737 T.I.T.S.U.P. but
after the C.R.A.P. was out of the way a C.U.N.T. system was introduced and proved
effective. And you thought YOU knew everything!
21/8/97: What a revelation. I thought all 737s crabbed left because Ansett and
Quaintass pilots were required to keep their wallets in their left pockets.