Avoiding overrun accidents on contamined runways
Landing on slippery surfaces requires precise touchdown in addition to judicious use of brakes or thrust reverse.
Landing performance on an icy runway. One can see that the thrust reversers contribute about 3 times more slowing-down effort than the brakes at high and intermediate speeds. In an emergency, keep the reversers until full stop, or shut down the engines if the compressors surge.
A headwind of 10 kts was used. The FAA-required landing distance (from Boeing references) is 5890 ft for dry runway and 6774 for wet-factored runway. (Note: These are zero-wind and no-thrust-reverse values!)
Using the FAA-mandated 50% of headwind distance correction (Title 14 CFR 25) would result in slightly shorter required dry and wet landing distances (DLDR and WLDR, respectively). The LDA of 7000 ft should then be quite satisfactory in both cases.
To reverse or not
Now, if the thrust reversers were not available, the BBJ landing on thin ice would exit the runway end at a speed close to 100 kts! In all honesty, it is rare for an entire runway to be covered by slippery ice, except maybe just after freezing rain.
Airports are doing a good job in cleaning and measuring braking friction, but these efforts are not conducted continuously, and in 10 minutes or less the runway condition can worsen significantly.
The charts on this page show the speed, deceleration and distances covered by a BBJ landing on thin ice. This is the result of a computer simulation developed by the author and published in the peer-reviewed AIAA Journal of Aircraft.
In this author’s opinion, today’s ICAO, FAA, JAA (etc) regulations provide too much insurance for landing on dry runways, while giving less insurance for operating on severely contaminated runways.
It is well known that—as many pilots, operators and industry experts have complained—to lump all contamination types into one “wet-runway” correction is not very sophisticated in the “GPS age.” I would suggest that demonstrated dry landing distance (DLD) becomes, say, 65% (instead of the existing 60%) of the DLDR (ie, 1.54 DLD).
At the same time, the surface contamination could be lumped into 2 groups (W1 and W2). Light contaminations (W1), such as rain on grooved runways, should continue to carry 15% correction over DLDR (W1LDR = 1.15 DLDR = 1.77 DLD), while the “heavy” contamination (W2), such as thin ice or wet snow, could carry, say, 30% correction over DLDR (W2LDR = 1.30 DLDR = 2.00 DLD).
This would give slightly less insurance for dry or moderately wet runways and a little more protection when landing on heavily contaminated runways. Of course, more research, data acquisition, modeling and statistical analysis are required to arrive at optimal “safety” factors.
To this author, the recent NTSB recommendation based on the Boeing 737-700 accident at MDW—that thrust reversers should not be credited toward stopping distance in the presence of runway contamination—makes little sense. We know that thrust reversing can be up to 3 times more effective than friction braking on slippery runways.
The absence of thrust reversing would thus require excessively long runways—and many parked airplanes. Instead, I would suggest designing more reliable, efficient and redundant thrust reversal systems that would offset the weakness of friction braking on contaminated runways.
Also, more industry efforts are needed to estimate surface braking conditions accurately and communicate them to flightcrew in a timely fashion. For flightcrews, it’s important to remember that, when landing on contaminated runways, accurate touchdown control and subsequent braking efforts are crucial.
Proper and timely use of thrust reversers is the most vital course of action when in stopping ground roll.
Nihad Daidzic is associate professor of aviation, adjunct professor of mechanical engineering, and chair of the Aviation Dept at Minnesota State University, Mankato MN. He is also president of AAR Aerospace Consulting located in Saint Peter MN.