Challenger Crashe at TEB

Ron McKinnon wrote:

From: "Matt Whiting"
Sent: Friday, February 04, 2005 1:57 PM

They only affect the aerodynamic behavior if they significantly affectt he
flow in the boundary layer. On some airfoils this a concern with fairly
small disturbances, but on others it isn't much of a concern at all.
Saying that you should not fly an airplane that has any frost on it is
just as silly as saying you should never take off with less than full
fuel. You have to know your airplane, nobody is arguing that. Most light
airplanes will fly just fine with polished frost on the wings and even the
control surfaces, I know my 182 did.


The issue isn't whether it will fly at all, but what happens to its
flying characteristics, and whether you're still operating
with safe margins for error. A few anecdotal cases where it
'flew', does nothing to answer that. There are many other
cases on record where it didn't.

My 182 flew just fine. I'm not aware of any 182 accidents from polished
frost and I know many people with many thousands of hours operating 182s
in my area and climate.


What, exactly is the expected level-flight stalling speed with
this particular amount of frost, 'polished' to this particalar
smoothness? Is it safe with the current loading, the current
density altitute, the current runway (the destination loading,
density altitude, and runway?) - do you have any safety
margin left? or have you used it all up by leaving frost on
the wings? or have you, in these particular circumstances
not affected it at all? What's the new stalling characteristics
of the stablizers, elevator and rudder effectiveness. Are you
going to be in the neigborhood of a tailplane stall on takeoff?
Is the frost layer uniform - does it affect the whole wing in the
same way?

You don't know. .

Same way you don't know after you fly through a swarm of bugs. Does
this freak you out also? It never bothered me. I can detect a stall
pretty easily in all of the airplanes I've flown. If the stall
commences at 60 knots instead of 50 knots, I don't really care, I just
stay above that airspeed.


And its not just about 'knowing your airplane'. How many
pilots can say with assurance how much contamination
will significantly affect the flow in the boundary layer?, or for
which airfoils its a concern with fairly small disturbances?
There are no doubt some that can, I'll grant you. But this is
not the usual rule. (Off the top of your head - What is the
airfoil on your 182? How sensitive is it to contamination.
How much contamination is too much on your 182.
What thickness of frost is too much for this airfoil? How
'smooth' *does* it have to be? And then what about the
stabilizers, elevators and rudder? Exactly what is the new
level flight stalling speed?)

I don't care exactly what it is as I mentioned above. The stall speed
varies constantly with loading and many other factors. If you fly based
on what the ASI tells you and not what the airplane is telling you, then
you are going to be a statistic some day.


The proposition isn't that you shouldn't fly an airplane that
has *any* frost on it - it is that you shouldn't fly it with any
frost on the critical surfaces: wings, stabilizers, rudder
(propellers).

And I disagree with that proposition and have done so successfully for
27 years. It just requires a little common sense and discussion with
people who have a lot of experience in your make and model.


It is obviously 'legal' in the US (the original FAR citation
shows this), so in the US you may obviously feel free to
use your discretion in the matter. But that doesn't necessarily
mean it is safe.

It is legal and safe if done properly. Just like most other aspects of
aviation. However, I'm not suggesting you should do it. I'm just
saying that for many of us it isn't a big deal.


In other places (Canada, for instance) it isn't even legal.

Then you shouldn't do it in Canada.


Matt