The present invention pertains to monitoring aircraft tire pressure.
The importance of maintaining proper aircraft tire pressure is illustrated by the fatal crash of a Bombardier Learjet Model 60 aircraft on Sep. 19, 2008, in Columbia, S.C. The investigation into the cause of the crash, and the circumstances and findings of the accident investigation are described in an article by Linda Werfelman, titled “Pressure Check,” published in the May 2010 issue of Aerosafety World Magazine (http://flightsafety.org/aerosafety-world-magazine/may-2010/pressure-check). In summary, multiple tire failures occurred during the takeoff roll, prompting the crew to reject the takeoff, but the airplane overran the runway safety area. The airplane was destroyed; two crew members and two passengers were killed; and the other two passengers were seriously injured.
The accident investigation found that the four main landing gear (MLG) tires had been installed less than one year earlier. The rated tire inflation pressure was 220 psi. Tests of tires of that type (Goodyear “Flight Eagle”) showed that the tires had a documented daily pressure loss of 2.2%, as compared to the maximum allowable tire pressure loss of 5% per day. Maintenance logs indicated that the plane had been flown on five days during the 12 days preceding the accident. Tests conducted after the accident indicated that the MLG tires were underinflated by about 36%. Interviews with personnel from all facilities that handled the accident airplane during the previous 12 days found that no one had serviced or received a request to service the MLG tires. The National Transportation Safety Board (NTSB) noted that the aircraft maintenance manual called for a tire to be replaced if it was operated with a pressure deficit of 15% or more.
On the Learjet 60, checking tire pressure was considered a job for maintenance personnel, not flight crew members, and required a technician to “crouch or crawl under the wing of the airplane to gain access to the MLG tire pressure valves requiring a person to lie on the ground to gain access,” partly because the landing gear doors may conceal the valves for the outboard tires. Nevertheless, instructions for “daily or regular” checks of tire pressure are included in instructions in the Learjet 60 maintenance manual and other Learjet and Goodyear tire maintenance publications, as well as Federal Aviation Administration (FAA) “Advisory Circulars.” Although pre-flight tasks for pilots typically include visually inspecting the tires for general condition (such as excessive wear, sidewall bulges, or visible tire cord), it is accepted that even dangerous underinflation may not be visible and can only be detected with tire pressure checks. For even properly functioning tires that have a pressure loss of, for example, 5% each day, it may take only a few days for the tires to reach an underinflation level below what the aircraft maintenance manual specifies for tire replacement. Consequently, instructions and/or regulations typically mandate that pressure checks be performed frequently, such as every three to five days, and such checks should be “cold checks,” i.e., checks conducted after tires have been at rest for a substantial period (at least two hours in the case of the Goodyear tires on the Learjet 60).