Re: Landing Gear Loads

Date:         02 Sep 98 01:08:21 
From:         Kim Hackett <hackett@southwind.net>
Organization: SouthWind Internet Access, Inc.
References:   1
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Landing descent veocities of 500 ft per second is way too high of a
number.  500 ft per minute is closer to the requirement.  Federal Air
Regulations Airworthiness Standards Part 25 for Transport Category
Airplanes is used to certify transport aircraft ranging in size from
business jets to the 747-400.  FAR 25.473 specifies the limit descent
velocity of 10 ft per sec at the design landing weight and a limit descent
velocity of 6 ft per sec at the design takeoff weight.  Under Part 25,
aircraft nose gear and main gear are drop tested to 10 fps limit (once in
a lifetime condition)  and 12 fps (ultimate) descent velocities.

For the Cessna Citation Excel, a 20,000 lb MTOW business jet aircraft, the
maximum nose gear (NG) static reaction sitting on the ramp is about 2,000
lb and each main gear (MG) maximum static reaction is about 9200 lb.
The gear is analyzed for a variety of landing conditions, including level
landing, tail down landing, one wheel, 2 wheel, and 3 wheel landing
conditions, and lateral drift landing.  For the Excel NG, the max vertical
dynamic landing load is about 9000 lb and the max NG drag load is about
4000 lb for a 10 fps limit landing.  The MG max vertical load is about
21,000 lb, maximum side load is about 8000 lb, and the maximum drag load
is 9100 lb.  These are limit values and are multiplied by 1.5 to get
ultimate loads for design.  The Citation Excel has one tire on the NG and
two tires on each MG.  The main gear tire loads would be roughly half the
main gear loads.

I don't know about a taxi length limit to control heat and tire flexure
during taxi.  I believe commercial jets have a taxi length limit that is
to keep the brakes from overheating.   I think that Boeing aircraft have a
minimum time limit between landing and takeoff which is to ensure adequate
time for brake cooling.  The brakes must be cooled down before the
aircraft takes off in case they need  the brakes in an aborted takeoff.