Re: [Phys-l] FW: [tap-l] Any Aviators on the list?

[Brian Whatcott <betwys1@sbcglobal.net>]

Somewhat informed speculation on weather effects: that icing effects are 
somewhat unlikely – that damaging lightning is somewhat unlikely – that 
meso scale updrafts could involve turbulence; that such weather is not 
uncommon in the area and that towering clouds to 50kft were present. 
This is provided at this URL:

http://www.weathergraphics.com/tim/af447/

You will realize that within two weeks of a jet crash you are quite 
unlikely to hear any cause offered on an evidential basis. So on this 
basis, I will offer some off the top speculation.
Small planes suited to aerobatics are designed to withstand loads of 6 
times the usual gravity loads, and with a 1.5 design factor, you can 
expect them to fail at or before a 9g loading. Utility light airplanes 
can be designed to meet less demanding loads, like
+ 4.5g. Large passenger jets are designed to meet still smaller loads 
which can be + 2 or 3 g. In part this is a facet of the unfavorable 
scaling effect of designing big.
You will recall the twin towers had slim structural margins for somewhat 
comparable reasons.
But there is also a favorable effect - that air loads are integrated 
over the large real estate - so a big gust may only affect part of a 
flying surface, moderating the structural demand. However, the current 
trend to efficient glass airframes has a side-effect: that when 
overstressed, it does not yield, it shatters, so that design factors are 
increased to account for this failure mode.
Every pilot is expected to enter adverse weather at a maneuver speed 
which balances the opportunity of an in flight stall, and at the fast 
end, an airframe failure due to full deflection of control surfaces.
Jet aircraft flying high also balance a limiting mach which can produce 
undesired aero effects like 'tuck', and stall speed which is a function 
of the same forces that drive the indicated airspeed. This particular 
aircraft type is among the most safe aircraft presently flying, so one 
expects an aggregation of two or three factors all conspiring to produce 
a fatal outcome. It is easy to mention some: pilot error of course - 
weather effects of course - and some aircraft system failure naturally.
The idea of in-flight breakup is encouraged when debris is reported over 
a wide radius.

Brian W



Sam Sampere wrote:
> John, this is right up your alley...
>
> Sam
>
> -----Original Message-----
> From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
> Behalf Of cbettis@unlserve.unl.edu
> Sent: Thursday, June 11, 2009 10:24 AM
> To: tap-l
> Subject: [tap-l] Any Aviators on the list?
>
> I am a bit puzzled by the Air France crash reports. It is being  
> suggested that frozen over pitot tubes caused erratic air speed  
> indications leading to loss of control. Wouldn't the pilots have  
> noticed this and used their GPS equipment to get their ground speed  
> and use it as a reference based on the difference between their last  
> good airspeed measurement and ground speed? Is it that ground speed  
> and air speed can be wildly different around thunderstorms?
>
> Cliff