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Engine Out? Establishing a Minimum Trim Glide

Your propeller has just stopped, and “things” just got very real! Your aircraft has been transformed from a powerful beast to a very heavy expensive glider. How you manage the next few minutes could be critical to your survival.

We were all taught very early in our training how to establish best glide airspeed in the hope that we will never actually have to establish this under pressure. It is a speed that every flight instructor expects you to know instantaneously and you have probably never been on a check-ride that doesn’t include at least one question about it somewhere.

The best glide speed is the airspeed at which an aircraft will travel the farthest for a given altitude, there is only one airspeed that will maximise glide range. A pilot's operating manual will provide that speed and perhaps a table or graph showing how far you can expect to glide from a given altitude above the ground. Most light aircraft glide two to three nautical miles for each 1,000 feet of altitude.

There are, however, many conditions that could affect the glide distance of your aircraft. A typical light aircraft must have its flaps up and the propeller stopped, which on a fixed-pitch propeller is almost impossible due to wind-milling. For aircraft with the variable-pitch propeller the lower the rpm the better, if possible, the prop should be completely feathered. The landing gear, if applicable, should be retracted. Anything still hanging out of the aircraft will induce drag and as a result shorten the glide distance considerably.

The maximum glide speed, as published in the pilot operating manual is calculated at the aircraft’s maximum gross weight, it is highly unlikely that the donkey will die on you when you are at max, as very few people ever fly with such a heavy loaded aircraft. The only time you can legally be at maximum gross weight is at take-off, and therefore the published best glide speed is too fast for almost all emergencies. The most common cause of engine failure is fuel exhaustion, “too much air in the tanks”, and if that happens the aircraft is already well below maximum gross weight.

To obtain maximum glide range, glide airspeed has to be reduced as the aircraft becomes lighter. Thus, making a “set in stone” maximum glide airspeed impossible to predetermine. The formula is a square root function of the actual gross weight divided by the maximum gross weight.

Now assume that you knew, or could compute the aircraft's current weight and have calculated the maximum glide speed for that weight. Can you extract square roots in your head? Especially in an emergency. Is that the speed you want to use? Maximum glide distance is also dependent on prevailing winds. Obviously, your gliding range is greater downwind than upwind. However, there may be an advantage to gliding more slowly with the wind, as the increased time airborne allows you to benefit from the wind for a longer time. Don't spend so much time figuring out how to maximise your glide distance that you overlook a perfect landing site right below you.

Of course, the opposite is also true, too, if you are bucking the wind with the glide. While upwind glide distances are always shorter than downwind glide, the maximum upwind range against the wind can be increased a little by increasing glide airspeed.

However, choosing the best-glide airspeed based on weight and wind does not complete the calculation. Your engine has stopped. Do you want to establish a maximum-range glide now, while attempting to restart the engine and deciding which direction to go for an emergency landing? Your initial goal might be to establish the airspeed that provides minimum sink this would keep the aircraft airborne for the longest time while you attempt to sort out the initial problem, which may be as simple as switching tanks and restarting.

It may be possible that all the fuel tanks are empty, do you want to attempt intricate calculations in an already high-stress environment? Do you accurately know the winds you will encounter on the way down? Of course not. And you might not be in a mental state to do them accurately, even if all that information was at your disposal.

A test pilot at Piper Aircraft Corporation developed a method as part of the FAA certification testing for each aircraft, he demonstrated a minimum trim glide. Almost every light aircraft will fly with power off and trim at maximum aft position without stalling. Practising this often may just save you when the dreaded “engine out” does happen for real. Please only practice this at a safe altitude.

Set the throttle to idle as the aircraft slows, gradually increase trim aft, initially keeping the aircraft at the same altitude. At some point, you will reach the published glide speed and let the aircraft begin a gliding descent. Note the position of the trim. It should be well aft, but not at the stop. Now gradually continue to trim aft all the way back. The aircraft slows down even further, but surprise it remains stable and does not stall! Take your hands off the yoke, and the aircraft continues to glide at a constant airspeed all by itself. You have now established a "min trim glide".

Now you know how to automatically establish a glide without worrying about the glide speed in knots, without fixating on the airspeed indicator, and without manipulating the elevator. You can now concentrate on troubleshooting the problem and if necessary search for the best place to land.

When actually needed in an emergency, the procedure is to simply continue aft trim to the stop at a smooth rate that does not cause the aircraft to balloon. There is no need to look at the trim indicator, as you can feel when the trim has reached the end of its travel against the stop. It is a waste of time to even look at the airspeed indicator. No input is needed on the yoke. At full aft trim, the aircraft descends in a controlled glide, it almost flies itself.

You can continue in that configuration all the way to the ground. However, when landing is assured there may be an advantage to re-configuring the aircraft for landing – flaps, gear, and more normal landing airspeed. But remember each of these actions increases the rate of descent and decreases glide distance.

A GPS is also a valuable tool in an emergency glide. It can point you to the nearest airport and tell you whether you are likely to reach that airport. Once established in a glide, the GPS will use your current ground speed, which reflects the prevailing winds, to indicate the time it will take to reach the emergency airport. Divide the altitude above the airport elevation (which is also indicated on most GPS databases) by the rate of descent indicated on the VSI. That will tell you the number of minutes of flight left. It should immediately be apparent if you will have sufficient time to reach the field.

Ideally, you want to reach an airport with 1,000 feet of altitude remaining, and at minimum of 500 feet to spare. Remember, in most cases, you will have to manoeuvre and reconfigure the aircraft when you reach the airport, and that will cost altitude. Resist the temptation to pass up a perfectly good field that will give you plenty of time to set up for a good landing, just because you might be able to glide to an airport.

Bad things happen when you least expect them, so stay vigilant and expect the unexpected.

Source - Dr. Ian Blair Fries CFI, senior aviation medical examiner, and ATP, Lear 35 type rating.



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