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<< Lancair Builders' Mail List >>
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I have just returned from investigating a nose gear failure of a IV-P (~550
Hours TT) on landing. There were no injuries and the plane suffered only
minor damage.
The incident occurred at Truckee (KTRK) airport with an 8000 foot density
altitude. The plane touched down on the mains and rolled for several hundred
feet before the pilot lowered the nose at approximately 95 KIAS (smoothly by
passengers accounts). The plane rolled approximately 100 feet before the
nose gear collapsed. Just before the plane stopped and under heavy braking,
the left main gear partially retracted and the left wing tip contacted the
runway surface. The passengers exited the plane without injury and
bystanders assisted in lifting the plane to allow the gear to extend fully.
Both flap and gear handles were fully down at the time of the incident and
the gear extension lights were on as witnessed by the pilot and co-pilot.
The following was found on inspection:
The prop and cowling had been removed for repair.
The mounting bracket at the aft end of the left main gear actuator had
failed, allowing the left main to partially retract and bending the actuator
shaft end and rack. The lateral braces were also bent and one had ruptured
at the flat head fastener.
The flap actuation push rod was damaged by the forward end of the rack.
The fitting at the bottom of the left caliper was damaged.
The nose gear was fitted with the original (thin) over center links and
there was approximately 0.025" radial play between the links and the nose
gear strut.
The nose wheel (Goodyear FC-II) was inflated to ~35 PSI.
The nose gear gas strut seemed relatively week.
Testing of the hydraulic system reveled that the HPU pressure switch was
intermittent, indicated by rapid on/off cycling of the HPU's motor.
Conclusion:
While no "smoking gun" was found the most likely explanation follows. When
the flaps were extended on short final the pressure switch failed to
activate the HPU and the accumulator pressure fell to near zero. The high
landing speed (8000' DA), play in the linkage, low tire pressure and the
tire profile of the FC-IIs together allowed the nose gear to shimmy. The
lack of sufficient over center force from either the gas strut or the gear
actuator resulted in the nose gear retracting. Heavy braking failed the main
gear actuator bracket.
I was discussing the incident with another high time Lancair IV pilot, Bob
Kelliher, and he recommended annual in-flight testing of the gas strut by
pulling the HPU breaker and performing a manual gear extension. If the nose
gear doesn't lock prior to operating the manual pump, replace the gas strut.
Good advice. Better advice may be to just replace the gas strut every 200
hours.
Regards
Brent Regan
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LML website: http://www.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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