Our
hangar group which includes retired airline pilots, an Applied Physics PhD and
a couple of engineering guys has kicked this issue of pattern stalls around
over the last few weeks. From that discussion I posit a theory for why
Lancair guys are getting killed in the pattern. Conclusion: it takes more
than a bad stall characteristic to get you killed. (You already knew
that.)
Fact:
our resident RV-6 owner/pilot reports that the early RV’s (before RV-7)
had a wing design that resulted in a sudden stall with big wing drop.
Sound familiar? The RV-7 which replaced the RV-6 has a completely
different wing profile to address this issue. However, RV-6s are not
falling out of the sky and killing people in the pattern.
Fact:
RV-6 wing loading is 1600 pounds on 110 square feet or 14.5 pounds per square
foot. A Lancair IV (short wing) loading is more like 3200 pounds on 98
square feet or about 33 pounds per square foot. When you fly an RV6 in
the pattern at normal pattern speeds (75-80 knots, flaps out) and pull back on the
stick turning final, it flies up. When you fly a Lancair IV around the
corner at 120 knots and pull up, it slows down – fairly rapidly –
because of the big change in angle of attack arising from high wing loading
(lots of momentum acting against a small wing panel).
Fact:
Flying at 120 knots in the pattern, the Lancair IV will have a turn radius that
is 2.25 times larger (1.5 squared) greater than the RV flying at 80 knots
(same angle of bank).
Now
let’s combine all these facts and propose an accident scenario. The
Lancair pilot enters downwind well behind slower traffic that is flying a
pattern in close to the runway. Mistake one: he is too close to the
runway. It is a busy day, lots of chatter, and he slows to his customary
120 knots, gear and flaps out.
He
then turns base, but his turn radius is 2.25 times larger than the RV he was
following for the same angle of bank. As he rolls out on base, he can see
that he will overshoot the runway centreline. Still lots of radio chatter
and he is watching the RV touchdown and roll out on the runway to make sure he
has room.
Having
crossed the runway centreline and while watching the RV and listen to the
chatter, he banks a little more steeply than normal on his turn to final and
pulls back, but he has not seen his airspeed indicator for maybe 15
seconds. A bit low to begin with, airspeed starts to decay rapidly. He
gets most of the way around the turn, the airspeed continues to
decay at an accelerating rate, he gives the stick a little more of a tug to
tighten the turn since he is now well past the centreline, and the airplane
stalls out of a 30-45 degree bank at 500 feet. End of pilot and
airplane.
Accidents
arise from a series of events. The events here were:
1)
Downwind
leg in too close following the “normal” traffic. Solution:
fly a very wide downwind regardless of where the slow guys are flying.
2)
Decay
of airspeed not noted after first turn or during second turn.
Solution: Eagle eye on the air speed indicator all the time, particularly when making
the turn downwind to base and then base to final. Limit to 15 degree
bank.
3)
Pulled
back on the stick after passing the runway centreline to tighten the turn while
at too low a speed. Solution: Never pull back on the stick or roll in
more bank angle to tighten the turn when in the pattern. Stick with the
15 degree bank limit in the pattern.
I
still find myself flying my downwind too close to the runway when following
traffic. It is force of a very old habit that must be broken. But
when I fly a big pattern and make a large U-turn from downwind to final with no
more than 15 degree bank, the margins at 120 knots remain large and it
all works out much more nicely with sped control being much easier.
The
RV 6 stall can be nasty, but it is not the nasty stall that kills Lancair IV
pilots. My theory: pattern in too close, air speed not watched, turn to
final pulled too tight because of the pattern air speed was allowed to decay and
pattern being too tight to begin with. Stall spin crash burn.
The
yellow alarm lights should go on when you are in close to the runway, or when
the cross wind blows you toward the runway on downwind. Be careful.
Your
comments welcome.
Captain
Tuna, Chicken of the Skies