On the Margin

January 5, 2011 by Bruce Landsberg

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!

Bruce Landsberg
President, AOPA Foundation

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10 Responses to “On the Margin”

  1. James T Lee Says:

    What is a “disruption in the space-time continuum”? Is this fancy jargon for “lost situational awareness” ??? Or what ?

    James T. Lee, MD,PhD
    AOPA Member

  2. Gerard Says:

    I had always heard that Jackson Hole (JAC) was a challenging airport, I guess that it is true. If people keep going off the runway then the runway might just be too short.

    I think your right about padding the margin whenever possible, but as pilots we still need to probe our limits and the limits of the machines we fly.

  3. Andy Young Says:

    James:
    I think “Disruption in the Space-Time Continuum” was meant as a joke. It’s a concept that comes up often in science-fiction, involving wormholes, time travel, etc. The space-time continuum, is, however, a real thing. It is a way of referring to the universe (and beyond, perhaps?) while including the dimension of time. This is necessary because it turns out that space and time are inseparable and influence each other. Einstein did a bit of work on this…
    Ok, guess I’m WAY geeking out here, far off the subject.

    Gerard,
    I would submit that runways are never too short; just that people keep trying to land with equipment and/or training that is inadequate for the runway they have chosen.

  4. Michael McGowan Says:

    Given the right combination of conditions the runway length is not the problem, no runway will be long enough! The worst problem in this and most overrun incidents/accidents is the often airline taught and mentality that once the wheels touch ground you are committed to landing and trying to stop. My airline had a MD-80 run off the end of the Cleveland airport and there was nothing wrong with the approach, speeds, touchdown point or equipment. No wheel spin-up due to icy runway mean no brakes, no thrust reverse and partial spoilers. In response to my input about going around, I was told the airline did not want that discussion as according to them rejected landing is a “test pilot” maneuver!

  5. Bruce Landsberg Says:

    James ….

    Andy has it right – it was intended as joke. You’ve probably not seen the movie ” Back to the Future” . In any case we’re tracking this mishap and have gotten a bit of inside information which is premature to release just yet. How’s that for suspense?

    Will advise when the time is right.

  6. Charles Lloyd Says:

    Bruce,

    Your add 900 feet for normal conditions seems to be similar to the Part 135 and 121 add 35% to the normal landing requirement to arrive at the minimum runway for landing calculations.

    This is not a bad recommendation.

  7. Bruce Landsberg Says:

    Charlie….

    You’re right — I haven’t had an original idea in 20 years !!! Should this become a recommended practice?

  8. Martin Says:

    Bruce,

    I just read two of your blog posts in a row. The first “A bad roll and equally bad commentary” had this admonition:

    —–
    •After an accident – don’t speculate on the cause or the talents of the pilot(s) involved.
    •All certificated aircraft have to meet specific standards and they are all controllable when flown inside the envelope. They are not controllable outside the envelope.
    •Unsubstantiated opinion, rumor and innuendo, while the staple of so-called news outlets these days is just that – DON’T feed the beast! GA will be better off even if you didn’t get a juicy soundbite on TV or in the paper.
    ——

    This post, “On the Margin” declares:

    ——-
    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.

    ——–

    So are you for or against speculation to the media (and I suggest your blog is a source for the general media) ???

  9. Bruce Landsberg Says:

    Martin….

    Fair question. We get plenty of direct media calls to AOPA to discuss these incidents and are very careful on how things are presented.

    Haven’t seen any indication that the media is paying attention to this blog and relative to the paragraph in question – there is not more credence given to one hypothesis or another. Most media outlets don’t like to admit that their experts are purely guessing — we freely admit it and advise that in good time the truth will out.

    Thanks for the note!

  10. Pete Ryan Says:

    off the thread? But the Air Force paid me and my home a visit this a.m. @ 3:30. I have a quicksilver, stored in my back yard. These folks were friendly enough, and professional as well. How ever, they had been looking on the ground and in the air for nearly 5 hours. about 8 souols in the van, and I never saw the plane?? I was still sleeping, as im sure, they would have loved to have be engaged in as well. It seems that my sons cat was doing cat things a couple nights agao, and in that endevour, the cat either landed on, or launched from the toggle switch on my 3.5 year old ELT. I felt like the idiot that i was for not removeing the power pack while the plane languishes in my back yard. pray for the Airforce and myself as well. thanks, and good night to you all. sun is riseing, and I still havent found a proper place to send my apology to the poweres that came in the night. PETE

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