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particular aircraft type was not an operating limitation unless otherwise stated, and
that it was not necessarily the aircraft’s maximum crosswind capability.
The information about maximum demonstrated landing crosswind published in the
operator’s A340 FCOM did not specify that it was an operating limitation for the
aircraft.
Crosswind landings
Crosswind landings are a routine occurrence. During crosswind conditions, a crew
must compensate for the drift caused by the crosswind to maintain the aircraft
21 FO is the abbreviation used by the AAIB for First Officer (copilot).
22 AAIB Bulletin Ref: EW/C2000/10/04.
– 19 –
aligned with the centreline of the runway during the landing approach. The factors
that limit an aircraft’s controllability in crosswind conditions include the
aerodynamic forces able to be exerted by the aircraft’s flight controls, and the sideforce
capability of its tyres.
Various handling techniques are used for crosswind landing approaches, including:
• sideslip (wing low)
• crabbed-approach to touchdown, with decrab during the landing flare
• combination sideslip and crabbed-approach.
Sideslip (wing low) approach
The sideslip (wing low) technique requires the crew to lower the upwind wing into
wind and to use opposite rudder to maintain the aircraft aligned with the runway
during the approach. During strong crosswind conditions, the control inputs required
to maintain the aircraft’s alignment with the runway may reduce the ground
clearance of the into-wind wing to the point where an engine podstrike or wingtip
strike may occur. Over-control of an aircraft about its roll axis by the use of ailerons
increases the risk of a podstrike or wingtip strike. That risk is a particular problem on
aircraft with wide-span wings fitted with underwing engines, such as the A340.
Crabbed-approach to touchdown, with decarb during the flare
The crabbed-approach to touchdown technique is flown in a wing-level attitude, with
the nose of the aircraft held slightly into wind to track the aircraft towards the
runway on the extended runway centreline. The angular displacement between the
aircraft heading and its track over the ground is the drift or ‘crab’ angle (figure 8).
– 20 –
Figure 8: Crabbed crosswind landing approach
Depending on the magnitude of the crosswind, during the landing flare, the aircraft
cockpit may be over the upwind side of the runway, with the aircraft’s MLG
straddling the runway centreline.
The ‘decrab’ during the flare is effected by the pilot applying rudder to align the
aircraft with the runway centreline, and opposite aileron to maintain the wings level.
The aim of this manoeuvre is to end up with the resultant aircraft momentum vector
along the runway centreline. The ‘decrab’ during the flare manoeuvre, however,
requires precise piloting skills, and the flare height must be correctly judged to
perform the manoeuvre successfully. In ideal conditions, the aircraft will generally
touchdown with little or no lateral drift. However, in strong crosswind conditions,
when the aircraft’s bank angle becomes the limiting factor in engine podstrike or
wingtip strike avoidance, an aircraft may land with a noticeable residual lateral drift.
Combination sideslip and crabbed-approach
In strong crosswind conditions, a combination of the sideslip and crabbed approaches
is normally the preferred handling technique, because it reduces the amount of
‘decrab’ required during the landing flare.
An aircraft crew maintains directional control of an aircraft with rudder after an
aircraft has landed. The maintenance of runway centreline tracking in crosswind
conditions, however, requires equilibrium between the aerodynamic forces from the
rudder, and from the mechanical forces generated by the aircraft landing gear and
tyres. The side force on an aircraft tyre depends on the groundslip angle of the tyre.
– 21 –
That angle is the angular displacement between the plane of rotation of the tyre
(generally approximating the aircraft heading), and the tyre’s direction of motion or
track over the ground (figure 9).
Figure 9: Touchdown MLG groundslip angle
Large groundslip angles result if an aircraft lands with a large drift or ‘crab’ angle in
strong crosswind conditions. Under those conditions, tyre side forces may reach a
‘saturation’ point at which stage the tyre will enter a fully-skidded state.
On dry runways, where good grip exists between the tyre and the runway surface, a fullyskidded
condition will normally occur at a groundslip angle of about 15-degrees. On icy
　
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