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14 Answers

Fastest, safe emergency descent

Asked by: 21777 views General Aviation

Suppose you have an on-board fire and want to get the plane on the ground in the least possible time.  What's the best maneuver for that, a steep glide, a tight spiral, or what?

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14 Answers



  1. Nathan Parker on Feb 08, 2012

    It will vary per airplane.  In theory, rate of descent is maximized when the product of drag and velocity is at a maximum.  So airspeed at redline, full flaps and gear down, with a 60 degree banked turn would probably be the fastest way down, but I doubt any airplane can do this.  With flaps and gear down, airspeed will be restricted to well below optimal.  Your choice will generally be between dirty and slow, or fast and clean, but a bank should always help you get down faster.
     
     

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  2. Brent on Feb 08, 2012

    There are a lot of different factors at play, including the type of aircraft, the type of fire, etc, etc. Your POH should have recommended procedures, and the AFM has a chapter on emergency procedures that includes fires and forced landings. 
    http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-7of7.pdf
     
    Another consideration might be what manuever is going to put you in a position to successfully complete a forced landing after you’ve managed to lose all that altitude. I’m not sure I would be able to transition from a red-lined 60 degree tight spiral to a short final approach to a safe landing zone.
     
    In the event of an engine fire, my instructor suggested a forward slip to get the maximum descent without structurally unsafe airspeed. This manuever may also help extinguish the fire.
     
    I suggest talking through various what-if scenarios with your instructor or fellow pilots. For example, what would you do if you detected an electrical fire in your wing (near your fuel tanks)?

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  3. Brian on Feb 08, 2012

    This is one of those impossible to answer questions. Regulations limit you to 60 bank and 30 pitch. So your fastest descent will likely be a clean spiral dive at 60 bank and 30 pitch, for reasons Nathan already covered — maximize drag.
     
    For a pilot who is well in tune with his/her aircraft and wearing a parachute, or operating under an emergency situation, or just plain willing to (stupidly) break rules — Assume normal category limit load of 3.8G you can fly a bank at 74.74 degrees level. In a steep descent this is complicated beyond my math skills. Long story short, and boring parts asside, something on the order of 80 degrees at Vne will likely give the fastest descent rate.
     
    The above is perfectly safe for a tenth of a percent of the pilots in the world. For me, and most of us reading, we’d probably rip the wings off. Boy that’d be an expedient descent huh? :-O
     
    In other words, pilot skill plays an immense role in what is conceivably ‘safe’. For the typical pilot, I’d venture to say 60 degrees bank and skirting with Vno limits would be best and relatively safe in dire situations. Just my humble opinion, however.

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  4. Jim Foley on Feb 09, 2012

    It appears that, according to the previous answers, that maximum drag/maximum airspeed should be used.  Why then, are steep spirals (commercial manouver taught to be used for emergency decents) performed at Vl/d +10kts and clean configuration (except gear down)?
     

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  5. Nathan Parker on Feb 09, 2012

    “Why then, are steep spirals (commercial manouver taught to be used for emergency decents) performed at Vl/d +10kts and clean configuration (except gear down)?”
     
    Because that’s not the purpose of the maneuver.  (And as an aside, Vl/d +10 isn’t the speed nor is gear down appropriate.)

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  6. Micah on Feb 09, 2012

    Brian, regulations aren’t constraints in an emergency (91.3). John, I think you should probably consider the type of emergency and type of pilot. I’ve never seen a steep spiral practiced with a student/private pilot. The forward slip is an easy maneuver to grasp and practice and should work well with a fire emergency.

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  7. Micah on Feb 09, 2012

    By the way, if you’ve never practiced a forward slip with you student, running near Vno with about -1,500 to -2,000 vertical descent in a C172, you should practice that on your next flight. That’s a skill that your student can understand and use.

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  8. Brian on Feb 09, 2012

    Micah, Hence why I listed it as an exception to the rule. 😉

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  9. Jim Foley on Feb 09, 2012

    Nathan, that’s how my flight school teaches it.  I guess it’s a time for a bit ‘o Google majic to learn the proper procedure.  Although, since the DPE I’ll be taking my Com. checkride with is also our CFI, so for now I’m gonna do it how he wants.

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  10. Nathan Parker on Feb 09, 2012

    “I guess it’s a time for a bit ‘o Google majic to learn the proper procedure.”
     
    Actually, you should reference FAA material.  The Airplane Flying Handbook just talks about assuming “gliding speed”, which isn’t necessarily best glide.  Best glide serves no purpose in this maneuver, since you aren’t trying to get anywhere.  Nor is the configuration specified.  As a practical matter, a clean configuration will enable you to get the three turns in from a lower altitude, so that’s what I choose.  A “minimum power speed” would also help you retain altitude, but that speed gets a lot closer to the stall speed for a 60 degree bank, so a good margin above that makes more sense.  This will probably get you closer to L/Dmax, but that isn’t a requirement.

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  11. Derek Schwalenberg on Feb 10, 2012

    You would rather go for the high-drag forward slip to get down as fast as possible over the steep spiral. The slip will keep the heat and smoke out of the cockpit much better than being coordinated and might even put the fire out. I suppose it depends on the situation though if its just a little electrical fire, shut off the master switch and if that stops the smoking go land normally at the nearest airport. (check for a light-gun if its towered and keep your eyes out for traffic either way like you always do)

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  12. John D. Collins on Feb 10, 2012

    As others have pointed out, the emergency descent procedure is aircraft make and model dependent.  I fly a Bonanza and there are several procedures used dependent on the model.  For my aircraft, a 1968 V35A, the procedure is to lower the gear, maximum RPM, and descend at maximum gear speed which is 143 Kts. The deck angle is wild, the drag is great, and descent is approximately 5000+ feet per minute.  So if I am at 6000 AGL with a fire, I will be in the flare a little over a minute later. My aircraft has a maximum flap speed of 113 knots and the flaps are of the continuous setting type without a detent, so the POH says not to extend the flaps.  Later models increased the gear speed to 152 Kts and have a flap control that has three positions, up, down, and approach.  Approach flaps can be used up to 152 Kts, so on those aircraft, approach flaps are used in the emergency descent procedure.  The combination of the added speed and approach flaps, make the procedure even more dramatic, and I routinely measure decent rates in excess of 6000 ft/min.  When I check out a new to the aircraft pilot, this is one of the maneuvers that is demonstrated.  You better not have any sinus condition.  It is difficult without continuous encouragement to get the pilot to lower the nose to achieve the emergency decent profile.  I know when we are near the proper deck angle when the seat rocks forward because the pilot’s weight goes over center.  It has to be demonstrated to be believed.
     
    If your POH has the emergency descent procedure in the emergency section, follow it.
     

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  13. Nathan Parker on Feb 10, 2012

    “You would rather go for the high-drag forward slip to get down as fast as possible over the steep spiral.”
     
    They aren’t mutually exclusive.  Full opposite rudder during a spiraling descent can make a good thing even better.

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  14. Derek Schwalenberg on Feb 13, 2012

    OK Nathan, never tried that. But I am going to next time I fly either by myself or with a commercial student.

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