Engine suddenly dies at cruise speed – how to achieve best glide?

13 Answers

1. 
   

   
   

   Bradley on Jul 15, 2014

   (b) maintain level flight until airspeed diminishes to best glide.

   The reason is that if you pitch up, you are increasing the angle of attack, this will result in an increase in induced drag. The increase in drag will decelerate the aircraft. You want to save as much potential energy as possible to transfer into forward movement.

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2. 
   

   
   

   LTCTerry on Jul 16, 2014

   Fully disagree with Bradley.

   There is only ONE speed that gives you maximum gliding distance. Any faster or slower results in a decrease. Maintaining level flight as you decelerate is not “saving” ANY potential energy (PE); it’s simply dissipating PE into heat (drag). During this time, the airplane is actually decending faster than it will once it reaches best glide (Best L/D).

   Remember your instructor telling you the first thing to do on loss of power is pitch for best glide speed?

   If you immediately pitch for best glide you will climb. Total Energy will remain relatively constant over a short period, so you convert kinetic energy (KE) to PE as you climb. Despite the power loss, you are briefly gaining rather than losing altitude.

   Once you have declerated to best glide, you have maximized the conversion of KE into PE and can use that additional few hundred feet to go even further gliding at best glide. In this scenario, the entire flight is either increasing altitude or descending at best glide; you are never decending at any airspeed less efficient than best L/D.

   Question: what is the difference between best L/D and minimum sink speed?

   Terry
   CFI-G

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3. 
   

   
   

   Mark Kolber on Jul 16, 2014

   And there we have it… the two sides of a discussion in which there is no best answer (anyone interested in a “pitch v power” discussion? 😀 ). That’s because the aerodynamic answer to “which will provide the farthest gliding range” in a single engine airplane really has nothing to do with “which should you do?”

   I personally prefer the “maintain level flight until airspeed diminishes to best glide” answer for reasons having nothing to do with supposed time increase from pitching up to get more altitude.

   #1 among those reasons is that (so far, anyway) every single-engine aircraft I’ve flown (about 30 make/models/variations from 2-seat LSAs to 6-seat Bonanzas) achieves best glide when simply holding a level cruise pitch attitude. I’ve done engine out exercises with students with the ASI covered. After running all the other various tasks and checklists, we then uncover the ASI. Guess what it shows.

   That means it’s just easy for the pilot to achieve best glide this way. In fact, if the pilot wasn’t taught best glide was something to “work on,” he or she would probably automatically hold that attitude because it’s so natural and familiar.

   It also means that the pilot can do other cockpit tasks, most importantly, finding a place to go, while the aircraft reaches, and the pilot trim for, best glide speed. That was brought home to me when I did a 172 checkout for a retired airline pilot. We were headed for a potential landing area before the airplane even slowed to best glide because his eyes were out the window with hardly a glance at the ASI.

   Assuming the “climb to best glide” people are correct about the aerodynamic gain, I really think that, with the exception of SuperPilot who never gets nervous and reacts correctly immediately without an “oh crap” moment of natural disbelief when the engine quits, any gain will be more than offset by the loss of the time taken to get to best glide in a nose-high attitude and making sure you don’t slow enough to induce a stall, instead of performing other important cockpit tasks.

   Besides, do you really want the first ingrained reaction to an engine failure to be “pitch up”? That can mean an early death if the failure occurred during the takeoff climb.

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4. 
   

   
   

   John D Collins on Jul 16, 2014

   I agree with Mark’s analysis. What the best plan is is highly situational. What are the wind conditions, where is your intended glide to point, is it in front of you or behind you. Making the best distance over the ground is not always the objective if you are going the wrong way. BTW, although best glide will occur at the same angle of attack and the glide ratio will not change when gliding in still air, the speed at which best glide (or L/D Max) varies with weight. At heavier weights, the glide speed will be higher than at lighter weights by the square root of the ratio of the weights. In my Bonanza, this varies between just under 100 Kts at light weights to 105 at GW.

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5. 
   

   
   

   LTCTerry on Jul 16, 2014

   Again, I have to disagree. It’s not the “supposed time increase from pitching up to get more altitude” that makes the the first answer wrong. It’s the concious choice to fly the airplane in an inefficient manner.

   Slightly rephrased the question asks “Will I glide farther if I glide at best L/D or if I fly faster than the best glide speed for a while.” Obviously, you will never fly farther if you purposly choose to fly at a less efficient speed. The question doesn’t include “what would you do in your airplane or what is the best plan?”

   The OP’s two scenarios are:

   a) Pitch from cruise for best glide ( and perhaps climb briefly) and then descend at best glide, or

   b) accept a worse than optimum descent rate until the descent rate gradually improves.

   He didn’t ask for pragmatic advice about how to actually respond.

   Mathematically, time spent at any speed that does not produce the best L/D will absolutely reduce the maximum “range.”

   From a practical point of view, if you have a headwind you should add half the wind speed; subtract half the tailwind (but never below min sink). But that’s all entirely a moot point. It’s only a valid calculation if you are going to make a long, straight final to the touchdown point.

   If there’s a suitable landing field within gliding range, then pehaps the minimum sink speed rather than best L/D is appropriate – it would provide more time to trouble shoot – endurance rather than range.

   Although I have less total flight time than the other responders, I do have over four hundred landings without an engine, including one in a Piper Arrow after a catastrophic engine failure.

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6. 
   

   
   

   Brian on Jul 16, 2014

   “There is only ONE speed that gives you maximum gliding distance.”

   If you wish to nitpick then it’s not one speed, it is one angle. If you’re a good enough pilot to pitch to that without any direct indication then my hats off to you. For me, I will choose whichever gives me the least amount of work. A zoom climb to dissipate speed rapidly is inefficient. So is flying at a cruise attitude until speed dissipates. The lesser of the two evils, in my opinion, is the one that requires the least amount of attentiveness as it allows focus on other tasks.

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7. 
   

   
   

   Mark Kolber on Jul 17, 2014

   As previously noted – the two sides of the question. One side favors zooming for altitude as more aerodynamically efficient and minimizes or disregards the effect of human factors; the other thinks less work by simple holding a cruise pitch* while finding a place to go is more efficient. It’s an argument of philosophy more than facts (each side disregarding the facts that don’t support its view).

   *Note, that’s not “maintaining level flight.” That’s “maintaining a level cruise pitch attitude.” IOW getting immediately to the pitch attitude that will result it best glide and that you will use all until landing is assured and gear and flaps deployed.

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8. 
   

   
   

   Kris Kortokrax on Jul 17, 2014

   Math would work OK if we lived in a theoretical world. We do not. I would take no comfort in landing in trees a half mile short of a runway that the math told me I should have been able to make. As a glider pilot, you know about thermal activity. An area of sink will throw the math out the window.

   I go along with Mark, John, Brian and Bradley.

   I would pitch to maintain my current altitude as speed dissipates. My glide would not commence until I had reached that speed. (I’m not descending, just because I haven’t pulled the nose up into a climb). This is easier to manage than trying to do the math to determine the best L/D speed for my weight at the time of engine failure, pitch up to attain that speed, anticipate the point at which I need to begin lowering the nose so as to not go below that speed and then maintain that speed.

   While the math may dictate one speed as optimum, I don’t know many pilots who could maintain that speed exactly. Also, best L/D would be a calibrated speed. The 172RG POH I have at hand shows best glide speed from 60-73 knots depending on weight. The airspeed calibration chart shows a 3 knot difference between calibrated and indicated at 60 KIAS and a 1 knot difference at 70 KIAS. I accept your statement that any deviation from best L/D will decrease glide performance. I also believe that it would be next to impossible for anyone to maintain best L/D exactly during a 6000′ descent, especially when attention is diverted to troubleshooting and attempting a restart. No one is going to be able to get the glide predicted by the math.

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9. 
   

   
   

   Bob Watson on Jul 18, 2014

   I can’t help but wonder what sort of difference we’re talking about. If one method extended your glide by a mile or two, that might be worthy of discussion. If it’s only a few feet (in theory), I’m thinking that other things would be more important.

   While you’re optimizing for best L/D, don’t forget the effect of weight and wind on the actual glide path. I need to dig out the books and brush up on their effects, but I know those are some real-world conditions that can make a lot of difference on where you’ll ultimately touch down. (e.g. if you have a choice, is it better to go for the downwind landing site or the upwind one? Knowing what factors have the most influence would help prioritize the decision making.

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10. 
    

    
    

    Brian on Jul 18, 2014

    Mark, “One side favors zooming for altitude as more aerodynamically efficient”

    This is exactly what I was trying to point out by mentioning that L/D is an angle of attack, not a speed. A zoom climb is not more aerodynamically efficient unless it is executed at exactly the L/D Max AOA. Pull too hard or not enough and you’re back to being inefficient, in theory. Without an instrument to tell you your AOA and a knowledge of your most efficient AOA this discussion is moot.

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11. 
    

    
    

    Al Roden on Sep 02, 2014

    In a 172, I just roll the trim to the nose-up limit and let go of the yoke – it takes the pressure off of flying the plane and let’s me concentrate on troubleshooting the issue.

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12. 
    

    
    

    PoqVaUSA on Mar 08, 2016

    From an engineering point of view, having excess speed for level flight kills off energy more quickly than using a 1-g climb to trade speed for altitude, and then transitioning to an optimal steady descent angle once the corresponding optimal speed is obtained.

    Drag is what takes away energy. Drag for a given drag coefficient is proportional to V^2, in terms of the dynamic pressure. Reducing the airspeed reduces the drag substantially. Climbing at 1-g should not require a substantial increase in angle-of-attack (vs. doing a >1 g pull-up), so the drag coefficient should not be much greater during the climb.

    This all assumes no changes in aircraft configuration (e.g. flaps). Piloting the aircraft (improving visibility for locating a landing site, etc.) always takes precedence to some theoretical consideration, but it\\\’s good to have the physics thought through before losing thrust.

    One site had the following posted, which seems to make sense:
    (Paul Wilson, http://www.pprune.org/private-flying/146029-best-glide-speed.html)

    I was taught to PANIC
    Persperate – get this out of the way quickly
    Aviate – set for best glide speed
    Navigate – choose landing site
    Investigate – try to get engine started again
    Communicate – Declare Mayday

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13. 
    

    
    

    PoqVaUSA on Mar 08, 2016

    Ok, one more thought. Maintaining altitude while slowing down until the best glide speed is a viable option, but once the optimal glide speed is obtained, it\’s absolutely essential to start the descent. Trying to climb or to maintain altitude for too long will result in slowing down to stall speed, which would just compound the problems for an already difficult situation. Setting up for a steady glide means keeping the angle-of-attack well below stall, so that\’s a key concern.

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