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On August 19, 2011, at 1555 eastern daylight time, a Piper PA-46-500TP, N133SR, was substantially damaged when it veered off the runway while landing at Manassas Regional Airport (HEF), Manassas, Virginia. The certificated private pilot and the passenger were not injured. Visual meteorological conditions prevailed. The airplane was operating on an instrument flight rules flight plan from Cobb County Airport – McCollum Field (RYY), Atlanta, Georgia, to HEF. The personal flight was conducted under the provisions of 14 Code of Federal Regulations Part 91.
According to the pilot, there was nothing extraordinary with the approach to runway 16L; he remembered glancing down at the airspeed and it being 92 knots about 500 yards from the end of the runway. He then added the last notch of flaps, and the airplane landed on the main landing gear, on runway centerline. The pilot kept the nose wheel off the runway 3 to 4 seconds; due to the length of the runway there was no need to force the nose down.
About another 3 to 4 seconds after the nose wheel touched down, the airplane started veering to the left. The pilot first added some right rudder, and as the nose did not correct, he added more right rudder. When the nose still wouldn't correct, he "jumped" on the brakes, and after that, put the propeller into Beta and reverse. After that, the airplane veered off the left side of the runway and into the median, where the nose landing gear separated from the airplane.
The pilot further stated that he did not land hard and had never had a hard landing in the airplane. The landing at his previous stop in Atlanta was similar, with no problems noted. The pilot did not know of any logbook entries where the previous owner had made a hard landing.
The passenger also noted that nothing eventful occurred with the landing, until "right after" the nose wheel touched down, the airplane veered off the left side of the runway.
The pilot reported 2,612 hours of flight time, with 130 hours in make and model. His latest flight review was accomplished on August 3, 2011, in make and model. The pilot also stated that he was familiar with the reversing characteristics of the PT-6 engine, and that he had flown a Beech C90A for the 6 years prior to the purchase of the accident airplane in June 2010.
According to the airplane maintenance logbook, the airplane's latest annual inspection was performed on July 29, 2011, at 1,903.6 hours of operation. During the inspection, the tube was replaced in the nose tire, the steering arm clearance was adjusted per the maintenance manual, and the nosewheel bearings were replaced, and the "assembly" was balanced.
Runway 16L was 5,700 feet long and 100 feet wide.
Winds, recorded at the airport at the time of the accident, were from 220 degrees magnetic at 4 knots.
WRECKAGE AND IMAPCT INFORMATION
On August 25, 2011, NTSB, Federal Aviation Administration (FAA), and Piper personnel examined the airplane. By that time, it had been recovered to a hangar and was up on jacks. All four propeller blades were bent opposite the direction of rotation, both wings and the fuselage exhibited impact damage, and multiple engine inlet vanes were fractured. The airplane's hour meter indicated 1,916.2 hours of operating time.
The nose landing gear wheel rotated freely, and the nose wheel bearings appeared pristine with new bearing grease.
The nose landing gear tire, an 8-ply Michelin Air 5.00-5, was found to be inflated to 55 psi. The right main landing gear tire was found to be inflated to 46 psi, while the left tire was found to be inflated to 47 psi. According to the Piper investigator, a fully inflated nose tire should indicate a pressure of 70 psi while fully inflated main tires should indicate a pressure of 55 psi.
An examination of runway 16L revealed two skid marks, about 200 feet in length on the runway surface. One of the skid marks commenced just to the left of runway centerline, and the other commenced just to the right of it at a distance consistent with positions of the nose landing gear and the right main landing gear. Both skid marks arced to the left, and exited the left side of the runway into the grass. Once in the grass, a third mark also appeared in the vicinity of what would be the left main landing gear. Shortly after its appearance, the mark attributed to the nose landing gear deepened into the ground, consistent with a nose landing gear having separated from the airframe at that point.
According to a Piper Customer Information Letter (CIL) dated March 27, 2008, early customers reported an occasional pull of the airplane to right "at the instant of touchdown." Although Piper personnel could not duplicate the events, a testing program combined with research into steering geometry resulted in two improvements to the airplane beginning in 2002:
- The 6-ply nose tire inflated to 50 psi was replaced by an 8-ply tire inflated to 70 psi, with the higher nose tire pressure reducing the size of the tire's rolling contact patch.
- A stiffer bungee was incorporated to increase the pilot's control authority over steering forces.
The CIL also stated that historical data validated the design changes. Prior to the changes, the rate of runway excursions per a given year divided by the number of airplanes in the fleet was 7.6 percent. After the changes, from 2002 to 2007, the excursion rate was consistent from year to year at 0.8 percent.
Airframe serial numbers prior to and through 4697125 had the design changes incorporated per Piper Service Bulletin 1106. Subsequent airframes had the design changes incorporated at the factory. The accident airplane serial number was 4697161.
The CIL further noted that a number of factors were identified as "critical" for optimal steering capability:
- Tire Pressure -
Low tire pressure can increase the forces associated with ground steering. Both Michelin and Goodyear indicated that aviation tires in service should have their cold inflation pressures checked daily, and for airplanes operating on a less frequent basis, inflation pressures should be checked before each flight.
The CIL further stated that according to both manufacturers, tires can lose as much as 5 percent of the inflation pressure in a 24-hour period and still be considered normal. The CIL provided an example: With a daily pressure loss of 5 percent, a tire that was inflated to 70 psi could drop to 49 psi in a week, and 15 psi in 30 days. The tire may still appear to be properly inflated but may be significantly underinflated. "Tire pressure should be checked by the pilot as part of the preflight inspection."
- Nose Gear Rake Angle -
The CIL also noted that another factor in steering stability is the relative (rake) angle between the lower (belly) skin and the nose gear strut, which should range from 90.0 degrees and 90.5 degrees. A rake angle outside the range "may result in dynamic instability, creating steering forces that exceed the pilot's ability to control."
"The nose gear rake angle is properly adjusted and verified at the factory and should be inspected and/or adjusted by a…licensed mechanic according to the schedule and instructions in the maintenance manual, any time the pilot experiences poor steering, or when there is replacement or service to the nose gear trunnion, the nose gear actuator, or the engine mount."
- Steering Arm/Roller Gap -
According to the CIL, two nylon rollers on top of the nose gear trunnion translate the rotation of the steering arm (from rudder pedal movements) into rotation of the nose wheel. When the landing gear is down and locked, the two nylon rollers are directly forward of the steering arm. Excessive clearance between the two rollers and the steering arm will result in loose steering and allow the nose wheel to point in a different direction than that commanded by rudder pedal inputs.
- Rudder Cable Tension -
The CIL also stated that low rudder cable tension can create variations between rudder angle and nose wheel steering angle. Operating with excessively low rudder cable tension could result in the nose wheel pointing in a direction that differs from the airplane line of travel at the instant of touchdown, even though proper pilot technique is followed.
The pilot noted that after the annual inspection, about every other flight, he would see a "Gear Warn" light sometime within the flight, but it did not occur on the accident flight.
The Piper investigator noted that if a landing gear actuator was not rigged properly in the up position, a drooping or mis-rigged condition could allow airflow to pull the door away from the gear warning micro-switch in flight, resulting in illumination of the landing gear warning light.