X WIND LANDINGS SENTRY
 

While the MLG on the Herc will withstand large vertical accelerations, any significant sideways thrust at touchdown is liable to damage it at best and knock it off at worst. Hence the wing down technique.

Best x wind aeroplane ever- 747.400. Drift remains on up to 40knots across and better if very slighty damp. (GRU)
Worst xwind aeroplane ever - DHC6 Twin Otter. Land across the runways into a taxiway if you can . (Unst)
Best twin jet B737-17R from Maersk Air on contaminated runways in Varga, Lulea, Umea, and the old Funchal (at night)

A few thoughts to add:

One of the main aerodynamic characteristics of interest is 'sideforce' which is the lateral force generated by sideslip; just think of the fuselage as a wing on its side. If you fly a wings level crabbed approach and then yaw the aircraft through the drift angle just prior to touchdown you generate sideslip, and the resulting aerodynamic force will generate a lateral acceleration across the runway. If the sideforce characteristics are strong, ie. a lot of aerodynamic force per degree of sideslip, a large lateral displacement across the runway will occur and at touchdown the landing gear will experience large sideways loads. In a tailwheel aircraft with the c.g behind the mainwheels any such lateral velocity will result in a tendency towards directional divergence, better known as a ground loop!

In a wing down approach the drift angle equals the angle of sideslip that is needed to align the fuselage with the runway. The bank angle required to maintain constant heading for a given sideslip angle is a function of the sideforce characteristics discussed above, the greater the aerodynamic sideforce produced, the greater the bank angle required. Therefore, high sideforce characteristics increase the possibility of a wing tip or wing-mounted engine pod strike on the runway. The bottom line is that, irrespective of technique, if sideforce characteristics are high then the crosswind limit will probably be low.

One of the other differences between a yawed, decrab landing and a wing down landing is that the former is a short term, transient manoeuvre following a 'normal' approach but the latter is a stabilised, steady state manoeuvre with sideslip applied. Therefore, if a wing down approach is stabilised early there may be drag generated, which will require more thrust than on a zero sideslip approach, and there may be a pitching moment requiring a different pitch trim setting and making the flare feel different. Therefore, a wing down approach may not be the same to fly as one flown in no crosswind.

The F4 was flown crabbed to touchdown because it had an aileron-rudder interconnect and very high lateral stability (apparent dihedral effect). If you had a crosswind from the right and flew a crabbed approach, if you applied left rudder to 'kick off drift' the aircraft would have a tendency to roll left and the large right stick input to prevent the roll would displace the rudder to the right, thereby reducing the left rudder input that you had made to align the aircraft with the runway.

In an aircraft with strong lateral stability such as the B747, it will roll markedly as you kick off drift. This will then require aileron firstly to stop the roll rate and then to correct the bank angle back to wings level. The combination of these two responses may result in full lateral control being inadequate. A good technique in aircraft with such a characteristic is to lower the into-wind wing slightly before the rudder is applied so that as the wing rises you can use aileron to arrest the roll rate and, if you judge it correctly, the bank stabilises at wings level.

On a wet, low friction runway, in a strong crosswind an aircraft may drift laterally across the runway due to the low sideways friction of the tyres attempting to oppose this. Therefore, landing with the crab applied gives an inertial vector which is straight down the runway, thereby reducing any tendency to drift downwind once on the ground.

It is all a compromise and in some aircraft more than one technique may work. Try to decide what is best for you and your aircraft. And beware of old, historical Pilots' Notes because the recommended technique may have been a 'standardisation' from the period and not necessarily the optimum technique.

Best technique I found doing several landings on a day with a 45 kt crosswind was wing down. The big advantage was that this was a stable condition and on touchdown there was no worry about drifting downwind. Any thoughts?

Thanks for neatly elaborating the technique listed as [2] in my earlier post. Never quite got to fly the B747, but lowering the wing immediately before de-crabbing certainly works well on the A320, on which I got more crosswind practice than on other swept-wing jets, and on which the wing-down technique is probably impracticable because of its FBW.

Yes, although I suggest that partial de-crab is still advisable, unless you are unfortunate enough to have to land on an icy runway. The FCOM of at least one type, IIRC, suggests half de-crab on a wet runway?

Thought that this was the "Military Aviation" posting page?