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respect to crosswind limitations as stated in the AOM, these are company limitations that are derived from information in the
AFM.
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Figure 1: Overview of maximum demonstrated crosswinds.
If the demonstrated crosswind is considered to be a maximum limiting value up to which it is
safe to operate the aircraft, the demonstrated crosswind value will appear as a limiting value in
the AFM. It is not allowed to operate the aircraft beyond this crosswind. The demonstrated
crosswind for practically all aircraft certified by the FAA and JAA was not considered as
limiting by the test pilots during the last forty years. Rare examples of modern aircraft with a
hard crosswind limit are the Fokker 70 and the SAAB 2000 (without aileron modification).
These aircraft were found to be demanding on aileron capacity, in particular after decrabbing
the aircraft just before touchdown. The test pilot makes a pure subjective assessment whether or
not the crosswind is limiting. The test pilot must consider that the aircraft is satisfactorily
controllable, without exceptional piloting skill or alertness, according to the rules of FAA and
JAA. This means that an average pilot should be able to handle the aircraft in those crosswinds
the test pilot did not consider limiting. During a crosswind certification flight, a pilot of the
airworthiness authorities accompanies the aircraft manufacturer’s test pilot. The airworthiness
authorities give the final subjective judgment regarding crosswind operations. When a test pilot
does its thing, he/she is well rested and prepared. The normal line pilot (e.g. arriving after a long
night flight) is not necessarily well rested and is not especially prepared to make landing in
strong crosswind conditions. Of course, it comes with the job description that he/she can do it!
The Flight Test Guide gives three methods of determining the crosswind during the test flight.
Only one is representative for what a line pilot has to its disposal, the tower wind. Okay you
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also have the FMS wind on the Nav Display ND. Is that wind not like the INS wind mentioned
in the Flight Test Guide? Unfortunately, the FMS wind is not what it appears to be, in particular
during takeoff and landing. First, the FMS wind is not corrected to a height of 10 meters. At 500
ft. above ground level the wind is about 50% higher than at a height of 10 meters. Second,
internal FMS calculation of especially the crosswind component during approach is filtered,
delayed and very sensitive for small errors in track or heading measurement. Furthermore, the
FMS wind is not corrected for sideslip. Therefore, when you decrab the aircraft the FMS will
give you a completely wrong picture. To quote the chief test pilot of the former McDonnell
Douglas Company, “you can get any crosswind you like from the FMS by simply side slipping
the aircraft”.
The same type of cup anemometers used to determine the tower wind, are generally used in the
portable ground recording stations. The big difference is that the tower wind not necessarily
reflects the actual wind you will encounter during the last part of the landing (just before
touchdown). In general there can be several sources for uncertainty in the tower wind reports, as
conveyed to the pilot. One of the main sources is obviously the stochastic character of the wind
phenomenon, due to which the wind may change since the last report to the pilot. Other sources
for uncertainty are the way the wind measurements are processed and the reporting procedures
to the pilot (which by the way are all according to international standards and recommended
practices for observing and reporting of surface wind characteristics!).
Depending on the way the data are analyzed crosswind derived from the INS system includes or
excludes wind gusts. At least one large aircraft manufacturer uses the INS data to derive a
crosswind by plotting the crosswind component as function of time. The crosswind at the time
the aircraft is 10 meters above the ground is then read off the plot. Engineering judgment is used
in fairing the data. Another manufacturer has a different approach in determining the crosswind
value during flight tests. During the flight tests the pilots of this aircraft manufacture requested
the tower wind when the aircraft was close to a height of 10 meters from the ground. The mean
wind given was then used to compute the crosswind during the crosswind certification flights. If
this last aircraft manufacturer had used the approach of fairing INS data as mentioned before,
the demonstrated crosswind capability for one of their aircraft would have been at least 10 knots
higher than presently mentioned in the AFM of this aircraft.
　
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