Solutions to the slippery problem of runway contamination
This month, the will begin a series of “contaminated” runway tests with a retired 727 at its William J. Hughes technical center in Atlantic City, N.J. Taxiing on 230-300-ft.-long “test strips” of 2-in.-deep “manufactured snow,” created by feeding ice blocks into a gasoline-powered chipping machine, the 727 will test a variety of new technologies aimed at providing pilots more information and training to deal with compromised surfaces.
In the U.S., much of the research stems from the December 2005 overrun of a-700 on a snow-covered runway at Chicago Midway International Airport. In Germany, the excursion of an Atlas Air -200 off the end of a snow-covered runway at Dusseldorf International Airport in January 2005 helped drive research and change in Europe. But despite nearly a decade of effort, runway contamination—how it is reported and how pilots plan for and respond to it—continues to be a vexing problem.
The manufactured snow testing at the Tech Center is part of a broad FAA plan to better understand runway surface effects, landing gear and tire physics and pilot human factors. In Atlantic City, engineers will measure aircraft tire-braking forces on ice- and snow-covered surfaces with the 727 as well as with a pavement load test vehicle—a wheel mounted on a gantry that moves over a simulated runway surface. The data will then be used to develop anti-skid system mathematical models to be studied in flight simulators, in part to “learn more about pilots' responses during operation on contaminated surfaces” and to evaluate different operational procedures “for accessing and processing responses from the aircraft systems,” says the agency. The 727 will also carry an intriguing new low-cost technology by Zodiac Arresting Systems to give pilots a real-time sense of brake performance.
Measurements taken by airports today include runway contaminant type and depth observations, ground surface vehicle friction measurements and pilot braking action reports, none of which correlate exactly to the performance an arriving aircraft will experience on the runway.
The German federal bureau of aircraft accident investigation, which last December published its final report on the Atlas accident, found that the “lack of a measurement method providing reliable braking coefficient values under all weather conditions” contributed to the accident. The braking-action values reported to the pilots were measured with an instrumented automobile driving along the runway. Those readings did not correspond with the actual runway conditions, in part because of the heavy snowfall and the imprecise mapping of friction wheel performance to that of an aircraft.
The International Civil Aviation Organization in 2012 published guidance to help airports better classify runway surface conditions in terms of friction measuring devices and contamination, but made it clear that a comprehensive solution is lacking. “It has been a long-sought goal to correlate the system response from a measuring device with the system response from the aircraft when measured on the same surface,” the guidance notes. “Substantial research” has led to new insight on the “complex issues taking place, but to date there is no universally accepted relationship between the measured coefficient of friction and the system response.” The FAA is leading several of the research activities. In the wake of the Southwest accident, therecommended that the U.S. regulations agency demonstrate the “technical and operational feasibility” of equipping aircraft with systems that “calculate, record and convey the airplane braking ability” to slow or stop on the runway.
Zodiac Arresting Systems' technology fits the spirit of the recommendation, and will be tested on the FAA's 727 imminently. Best known for building the crushable concrete overrun pads now in place at 81 airports globally, the company is seeking to broaden its safety portfolio.
The system is simple in concept: Correlate the command that the pilots make to the brakes—either through the rudder pedals or the auto-braking system, with the command that the anti-skid system actually sends to the brakes—and assess the runway conditions. If the difference between the pilot command and the anti-braking command is greater than a certain threshold, trigger an alert to the pilots, possibly similar to a low-level wind shear alert. That same information, augmented with the position on the runway where the measurement was taken, as well as other data, would then be sent via a digital communications path to other aircraft and the airport.
The FAA trials are meant to validate the concept, and assuming success, the company will be searching for a launch airline to help with the certification effort for retrofit and forward-fit versions of the safety aid.