Saturday, October 8, 2011

Finessing Failure

Engines fail, and pilots since the Vickers Vimy have worked hard to handle this safely.

A standard part of multiengine training is the drag demo. You pick a reasonable airspeed and set the power to fly level. Then, put the landing gear down; after a few transients, the airplane settles into a steady descent rate. "Write that down: gear down is 500 feet per minute," or whatever.

After you do the same thing with flaps, you go on to see what happens with a windmilling propeller, and finally with a windmilling propeller in the zero-sideslip configuration. (In a multiengine airplane with asymmetric thrust, flying with the wings level amounts to a mild slip, as a yaw string will clearly demonstrate.)

The student gets a lot of good knowledge out of this demonstration: windmilling propellers produce a lot of drag (in most multis it is about the same as the landing gear), and zero-sideslip really improves performance.

Too bad we're teaching the wrong thing.

For the past little while I have been rethinking all of the problems of aerodynamics from the point of view of two parameters, essence and finesse. (I've written about this here and here, among other places.) Briefly, essence is composed of all of the energy (potential, kinetic, chemical, intellectual, emotional) available to the airplane. Finesse is simpler; it's simply the glide ratio at the current speed, something a glider pilot learns from the polar.

So what does a multiengine pilot faced with a failed engine need to know? The descent rate is in feet per minute, which might be useful if the pilot wants to be able to stay aloft until, say, the earthquake ends. But that's not usually the concern; the concern is Can I make it to the airport?, and the proper number to know is measured in feet per foot; it's finesse.

The thing is that finesse is quite easy to calculate, even among those who don't like to calculate. The key fact involved is that 1 knot is very close to being 100 feet per minute, so the vertical speed indicator pointing to 600fpm is also pointing to 6 knots. So, to find the finesse, divide the airspeed by the VSI reading. Done. You can refine this by considering true airspeed, but this way your estimated glide ratio is lower, ie, more conservative.

Ground speed is another matter, so if you want the best effective finesse divide GPS groundspeed by the VIS reading.

It is worth remembering that a 3 degree descent angle is a 20:1 glide ration; that's the same fact as the fact that the target descent rate for an ILS approach is 5 times the groundspeed.

The essence of an engine failure is more complex, no matter how many engines there are. One thing is sure: panic makes no essence available, which is why regular practice is essential.

One other thing: with an engine failure the chemical energy -- fuel -- that the airplane carries become ballast, ballast that burns. Be careful.

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At October 8, 2011 at 11:05 PM , Blogger Alex Roetter said...

Interesting post, thank you. One clarifying question: You say that a 3 degrees ILS descent is 20:1 (follows from the tangent of 0.05)

You then say that is equivalent to saying a target descent rate for an ILS approach is 5x groundspeed.

Can you clarify the units? AFAICT, it should be one twentieth the ground speed, from above. My intuition checks out in the following example: for an 100 kt ground speed, one twentieth is 5kts, which by your rule of thumb above is 500 fpm, a perfectly reasonable descent rate for an 100 kt precision approach.

Thanks for clarifying in advance. Love the blog!



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