The fatal runway overrun accident of a Learjet 60 on Sept. 19, 2008, in Columbia, S.C., has generated a flood of information about the dangers of low tire pressure and aborted takeoffs. According to the NTSB, “The probable cause of this accident was the operator’s inadequate maintenance of the airplane’s tires, which resulted in multiple tire failures during takeoff roll due to severe underinflation, and the captain’s execution of a rejected takeoff after V1, which was inconsistent with her training and standard operating procedures.”
According to the Safety Board, the accident airplane’s tires lose 2 percent of pressure every day. The full inflation of 219 psi drops to 185 after eight days and after three weeks reaches 140 psi, the level estimated in the accident airplane.
During the takeoff sequence, the outboard right tire burst first at 137 knots followed by the other tires, due to excessive flexing of the tires and heat damage caused by underinflation. Tire fragments damaged the squat switches, and the logic switched to air mode, which automatically stowed the thrust reversers and resulted in high forward thrust when the pilots were trying to slow the airplane during the rejected takeoff. Both pilots and two passengers were killed after the Learjet crashed into an embankment and caught fire. Two passengers were seriously injured.
The NTSB concluded that the insufficient tire pressure was a result of the operator’s inadequate maintenance, which raises questions about who is responsible for tire pressure and how pilots can assure that tires are properly inflated.
In an NBAA podcast about this issue, reporter Pete Combs interviewed Keat Pruszenski, manager of customer support engineering at tire manufacturer Michelin North America. Michelin recommends checking tire pressure daily or before each flight for aircraft that don’t fly every day. Pilots are responsible for airworthiness, Pruszenski said on the podcast. He then asked: how can pilots assure airworthiness if the tire pressure isn’t checked frequently?
Crane Aerospace manufactures a wireless tire-pressure monitoring system that uses tire pressure sensors built into the fill stem and a wireless reader device that records tire temperature and pressure. This system will be available shortly for Cessna and Learjet models, according to Nathan Smith, business development manager in Crane’s sensing and utility systems division, who was also interviewed for the NBAA podcast. Smith warned that checking tire pressure using a traditional gauge may be risky because the tires lose pressure each time the gauge is used.
Tire manufacturer Goodyear also recommends daily tire pressure checks. Both companies recommend using dry nitrogen to fill tires and calibrated gauges when checking pressure. More information on Michelin and Goodyear aircraft tire maintenance practices is available on each company’s Web site (www. airmichelin.com and www.goodyearaviation.com).
FAA materials on the subject of tire pressure include Safety Alert for Operators 09012 and Advisory Circular 20-97B, “Aircraft Tire Main- tenance and Operational Practices.”
Can Pilots Check Tires?
In its analysis of the Learjet 60 accident, the NTSB expressed the concern that the FAA isn’t clear about who is permitted to check tire pressures. In fact, there is no regulation that prohibits pilots from checking tire pressure. While tire pressure checks are not specifically listed as preventive maintenance that pilots are allowed to perform under FAR Part 43 Appendix A, the assumption is that because this rule allows pilots to change tires, then they automatically must be allowed to fill the tires with air. If filling a tire with air is a maintenance procedure, what about simply testing a tire for pressure?
Learjet vice president and general manager David Coleal on Jan. 8, 2009, sent the FAA a request for legal interpretation, asking whether the pilot of a transport-category (Part 25) aircraft operated under Part 91 or 135 can check tire pressure. At issue is whether checking tire pressure counts as maintenance, preventive maintenance or a preflight item. Pilots can obviously accomplish preflight items and are allowed to perform preventive maintenance on aircraft operated under Part 91, but not under Part 135, unless the operator obtains an exemption. A mechanic is required for Part 135 preventive maintenance or any item labeled as maintenance. (The FAA also determined that the preventive maintenance rule allowing pilots to change tires applies to pilots changing tires on a small general aviation aircraft, not on a transport-category jet.)
In the letter to the FAA, Bombardier explained that “Bombardier Learjet’s engineering and pilot specialists believe ample precedent exists for ‘qualified pilots to safely perform tasks that require mechanical, physical interaction with the airframe under the umbrella of preflight checks.’”
But the FAA responded that the Flight Standards Aircraft Maintenance Division determined that “checking tire pressure on a Learjet Model 60 aircraft is preventive maintenance and not a simple preflight task.” The reasons for this determination include “the high tire pressure (up to 219 psig), the need for a proper and calibrated gauge and the possibility of an incorrect reading if the check is not performed properly.” Thus, in the FAA’s view, a pilot of a transport-category jet operated under Part 91 may check tire pressures, but not a pilot operating under Part 135. Yet confusingly, the FAA says that same Part 91 pilot may not change that jet’s tire.
It should be noted that all maintenance, including preventive maintenance, must be logged in the aircraft’s logbooks, and this clearly includes tire pressure checks, according to the FAA’s interpretation.
When To Check Tires
According to the NTSB, “Some operators are not sufficiently aware of the appropriate tire pressure check intervals for the airplanes in their fleets and are operating their airplanes with tires inflated below the aircraft maintenance manual replacement specifications.
“Aircraft maintenance manual (AMM) formats that refer to tire pressure checks as guidance information rather than required maintenance intervals and the lack of standardization of AMM formats with respect to the location of tire pressure check interval information do not provide sufficient emphasis on the criticality of checking and maintaining tire pressure.”
The FAA did ask Learjet in November 2008 to revise the AFM to require daily checks
of Learjet 60 tire pressures.
In March 2009, Learjet issued a flight manual change, with a system limit specifying that nose and main tire pressures “be checked within 96 hours (not flight hours) before takeoff using the procedures listed in Chapter 12 of the Learjet 60 Maintenance Manual.” In the “normal procedures, exterior preflight section,” items were added “to check nosewheel and main tire pressures.”
Continuing the Takeoff
No matter how well maintained the aircraft, there is always the possibility that a tire can blow during the takeoff run. In the Learjet 60 accident, the NTSB faulted the captain for not continuing the takeoff and trying to abort at a speed greater than V1. A former Learjet instructor told AIN, “When I instructed at FlightSafety International, we cautioned about aborting with a blown tire, because you are dependent on the tire footprint for stopping power. With just one blown tire you lose 25 percent of your footprint. It sure seems as if [the crew] could have gotten the aircraft in the air and kept it there, there would have been a better outcome, even if tire debris [shut down] one of the engines.”
A blown tire appears to have been a factor in another accident, although this one didn’t involve fatalities. On Jan. 8, 2010, a Falcon 20C registered in Mexico was substantially damaged after it ran into deep snow after an aborted takeoff from Eagle County Regional Airport in Colorado. The NTSB investigator interviewed the airport manager and captain of the Falcon, who said that the left main tire blew during the takeoff roll. “The captain was unable to stop the airplane in the distance remaining. Both landing gear collapsed and the right wing was buckled.” The NTSB has not yet released a final report on this accident.