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runways, a fully-skidded condition may occur at a groundslip angle of only 5-degrees. It
is for that reason that crosswind landings on contaminated runways pose greater risk than
on dry runways.
The MLG on an aircraft fitted with a conventional tricycle undercarriage is located
aft of the aircraft centre of gravity. During a crabbed landing in crosswind
conditions, the MLG tyre side forces tend to ‘decrab’ the aircraft into alignment with
the vector of its forward momentum as depicted in figure 10.
– 22 –
Figure 10: MLG tyre side forces during ‘decrab’ after touchdown
Angle of bank and ‘crab’ angle requirements
The aircraft manufacturer published a series of Flight Operations Briefing Notes
(FOBNs) that provided:
an overview the applicable standards, flying techniques and best practices,
operational and human factors, suggested company prevention strategies and
personal lines-of-defense related to major threats and hazards to flight
operations safety.
The FOBN affecting crosswind landing techniques included advice that higher
crosswind conditions were typically those where the crosswind component exceeded
15 to 20 kts and that, under those conditions. ‘…a safe crosswind landing…’ would
require a crabbed-approach with a partial ‘decrab’ before touchdown. The FOBN
included information that on most Airbus models that would require a ‘maximum’ of
5-degrees ‘crab’ angle and 5-degrees of bank angle at touchdown.
– 23 –
The FSF ALAR Toolkit23 also provided information on bank angle and ‘crab’ angle
requirements for crosswind landings.
The FSF briefing note included information that, with a steady 30 kt crosswind, a
sideslip approach would require about 9-degrees of into wind bank, and that such a
bank angle would place an aircraft close to its geometry limits at which an engine
podstrike or wingtip strike could occur. Additionally, that bank angle would place an
aircraft closer to its rudder/aileron authority limits.
Under the same crosswind conditions, a crabbed-approach to touchdown (without
‘decrab’ at touchdown) would require a 13-degree drift or ‘crab’ angle, and would
possibly result in MLG damage at touchdown.
If a combination sideslip/crabbed approach was flown, it would require only 5-degrees
of bank angle and 5-degrees of drift or ‘crab’, increasing safety margins relative to
geometry limits and also rudder/aileron authority limits.
The FSF concluded that for most transport-category aircraft:
…touching down with a five-degree crab angle (with an associated five-degree
bank angle) is a typical technique in strong crosswinds.
Aircraft tyre loads
Centrifugal forces
A combination of heavy loads and high operating speeds results in strong centrifugal
forces acting on an aircraft’s tyres. In that case, the peripheral shape of the aircraft’s
pneumatic tyre(s) is deflected by loading from the tyre’s contact with the ground.
That deflection can occur during landing and takeoff.
During rotation for takeoff, the portion of the tyre leaving the ground tries to resume
its normal unloaded shape. However, centrifugal forces acting on the tyre, along with
inertia, mean that the tyre tread does not stop at the normal peripheral position, but
overshoots that position and so distorts the tyre’s natural shape. That distortion
continues around the circumference of the tyre tread as it rotates, in what is termed a
‘traction wave’. The traction wave becomes more pronounced as tyre rotation-speed
increases.
The Goodyear Tire and Rubber Company have published figures to help explain the
magnitude of forces acting on an aircraft tyre at rotation.24 Goodyear calculated that the
traction wave deflection of a tyre rotating at 4,200 RPM was 4.82 cm. The 4.82 cm
outward radial deflection was equivalent to an average radial acceleration of 60,960 m/sec ,
which was in excess of 6,000 G. Therefore, the tyre tread was experiencing between 200 to
266 oscillations per second.
23 Flight Safety Foundation Approach-and landing Accident Reduction (ALAR) Toolkit – Briefing
Note 8.7 – Crosswind Landings.
24 Aircraft Tire Care and Maintenance, (2002) Goodyear Tire and Rubber Company.
– 24 –
Tensile, compression, and shear forces
In normal operating conditions, aircraft tyres are also subjected to high tensile,
compression, and shear forces. In an unloaded tyre, the internal tensile forces acting
on each layer of fabric in the tyre are uniform. However, when a tyre becomes
deflected under load, the tensile forces on the outer plies become higher than those
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