Re: Birgenair 757 crash - A&C article

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         02 Jun 96 16:23:50 
References:   1
Followups:    1
Next article
View raw article
  or MIME structure

In article <airliners.1996.605@ohare.Chicago.COM> (Francis JAMBON) writes:
>What I do not understand is if a pitot probe is blocked. the indicated
>speed must be lower than the air speed, not highter ?

Higher.  An airspeed indicator basically measures the difference between
static pressure and total pressure.  These inputs come from two sources:
static pressure from a static port, and total pressure from a pitot tube

Let's assume that:

P_stat = static pressure
Dynamic pressure, q, is 1/2 rho * V^2, where rho is density and V is the
	true airspeed of the airplane.
Ptot = q + Pstat.

So the airspeed, v, is roughly proportional to
	Ptot - Pstat.

If total pressure is held constant, then you'll note that *a* static value
is trapped in the Ptot term, such that the steadily decreasing static
pressure in the second term, as the airplane climbs, will result in a
steadily increasing airspeed.

So this results in velocity being proportional to:
	P_stat_blocked - P_stat_ambient

Where P_stat_blocked is the static pressure when the tube was blocked and
P_stat_ambient is the current pressure.

You may want to plug some numbers into this:

	V = sqrt(2*(Ptot - Pstat)/rho))


	V = sqrt(2*(P_stat_blocked - P_stat_ambient)/rho))

Just remember to keep the two static terms (the one that goes into the
velocity calculation and the one that goes into the Ptot calculation)
different, since they will be different in this case (the value in Ptot
will be stuck at the value present whenever the pitot tube got clogged).

This explains why, as the airplane seems to accelerate, the pilots pull
further and further back on the stick, in a futile attempt to slow down
on the airplane, until the airplane eventually stalls out.  By then,
they're at such a high attitude that recovery becomes somewhat doubtful.

>Francis JAMBON

Robert Dorsett                         Moderator, sci.aeronautics.simulation