gliding back to your departure airport

"Harold" wrote in message
...
[...] If
my best glide is 85 KTAS and it loses 700 fpm at that speed, shouldn't I
be
guaranteed I can make it back if I climb at 84 KTAS and 701 fpm ?

Do you descend at 700fpm gliding at 85 knots (which you should reference as
*indicated* airspeed, not true) with the wings level? Or did you verify
that descent rate in a turn?

Several factors prevent the simplistic analysis you've made from being
valid:

* The turn itself increases descent rate

* You need to turn a net of closer to 270 degrees: 225 to get you on an
intercept course back to the runway, then another 45 degrees the other
direction to align yourself for touchdown.

* Typically you are climbing into a headwind; that becomes a tailwind
halfway through your turn and through the remainder of the descent. The
tailwind will either push you past the runway, or you need to steepen your
descent by increasing the descent rate. Either way, that interferes with
the basic "if I climb at such-and-such a rate, then instantly turn 180
degrees and descend at a different rate, can I make it back to the runway"
simplification. Assuming "no wind" conditions doesn't make sense, because
that assumption is almost never correct and the consequence is significant.

Don't forget the reaction time it takes to start the turn, and the time
spent at something other than best glide airspeed. For the vast majority of
pilots, a large proportion of the post-engine-failure flight will be done
quite a bit away from optimally.

If you have a hard time believing this, it's easy enough to experiment.
Find yourself a nice quiet airport where you can depart straight out. Climb
straight out to 2000' AGL, then cut the power. Wait a second or two (since
you won't be surprised by the power cut), then go ahead and start your turn
back to the runway. Note the altitude loss at the point at which you are
back aligned with the runway. This will give you the absolute *minimum*
altitude you might successfully attempt such a turn-back.

For extra credit, time the post-power-cut flight, noting your airspeed as
well. This will allow you to figure out how far you actually flew during
the descent, which will give you an idea of whether you'd have actually had
enough runway left to land on by the time you got all set up. For extra
accuracy, take someone along to keep track of the actual airspeed, or use a
GPS to track the experiment (to get distance directly, rather than depending
on speed over time).

Finally, keep in mind that not all airplanes have the characteristic yours
does. In fact, I'd say it's unusual to find an airplane that climbs and
descends at exactly the same airspeed and vertical speed. Especially
powerful aircraft will climb more steeply than they descend, while slower,
lower-powered airplanes will climb less steeply than they descend.

Pete