Archive for January, 2011

On the Margin

Wednesday, January 5th, 2011

What happens when we operate an aircraft at or close to the margin? Most of the time it works. This can lead to some not-so-desirable thought processes because safety may to be compromised. But hey, it always worked before.

Margin is -”An amount allowed or available beyond what is actually necessary.” Other definitions include “To be on the border or edge” and for economists it’s “The point at which the return from economic activity barely covers the cost of production, and below which production is unprofitable.”

This concept of margins may apply to a recent American Airlines incident in Jackson, Wyoming (JAC) where, according to the NTSB, “At about 11:38 am MST on Wednesday, December 29, American Airlines Flight 2253 a Boeing 757-200 (N668AA) inbound from Chicago O’Hare International Airport, ran off the end of runway 19 in snowy conditions while landing at Jackson Hole Airport. No injuries were reported among the 181 passengers and crew on board. The aircraft came to rest in hard packed snow about 350 feet beyond the runway overrun area. An initial inspection did not reveal any structural damage to the aircraft.”

There have been a number of runway excursions at Jackson Hole (KJAC), some involving runway contamination, but there have been other mishaps as well . You can check out the light aircraft list here. Runway 19 is 6,300 feet long, 6,451 feet MSL with a .06% downslope and is served by an ILS and PAPI. Additionally, it has a porous friction course overlay. So-a normal landing turned into a slip slide. Ironically, the Air Safety Institute was recently engaged in a meeting with FAA and some airline operators discussing the abnormally high rate of runway overruns at Jackson Hole. The discussion was about margins, runway length and equipment capability. The airlines obviously felt that things were sufficient and it should be noted that there are thousands of flights operated into KJAC without incident. The question is how far to go into the margin – what equipment is required to be operational, etc?

What happened last month? The possibilities are myriad but let’s speculate: hot and/or high on the approach, delayed deployment (or non-deployment) of thrust reversers and spoilers due to pilot or mechanical issues, braking problems, a tailwind – something else? We’ll soon find out.

Now apply this to GA flight ops. Would you fly into or out of an airport that was on the margin? Sure – do it all the time!!! In GA, we are lead to believe this is a good idea because the POH says the aircraft is capable of it and the FAA tests to this level on the knowledge test, even asking for interpolations to the nearest 200 feet. What lunacy! Go to the next worst case and use that.

Whatever the engineering figures are in your POH for takeoff and landing distances, add 40 -50% under normal conditions. If the book says the aircraft can get over the 50 foot obstacle and stop in 1870 feet – add 900 feet for a real world runway length of about 2,700 feet. Saves brakes, tires and reduces pucker – all desirable even if your aircraft isn’t the perfect physical specimen used for the POH number. If it’s wet or slippery, double the number, or in case of ice, snow banks or crosswinds it may be that the cost of production just isn’t profitable. Operating on the margin in aircraft isn’t any smarter than it is in economics and we’ve certainly seen enough of that recently!