Re: Peruvian 757 crash -- possible cause reported

Date:         23 Nov 96 03:36:23 
From: (Douglas Swisher)
References:   1 2 3 4 5
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> Praxis - Bjxrn Erling Flxtten wrote:
> > However, theory aside, isn't it amazing that modern aircraft with all
> > this technology is not equipped with a sensor to check for obstructed
> > static and / or dynamic ports?
> The 757/767 does display ground speed on the EADI and wind speed and
> direction on the EHSI. These numbers come from the IRS.
> Seems like this information (along with pitch attitude and vertical
> speed) should be enough to keep you out of harms way.
> BTW- VS also comes from the IRS on the 757/767.

If I am not mistaken the wind speed is in essence derived from
the vector operation of the difference between the true airspeed
vector and the ground speed vector.  True airspeed is derived from the
calibrated airspeed, heading and outside air temperature.  Only
ground speed comes from the IRS.  There is an inertial vertical
speed in the IRS but I would guess that the vertical speed displayed
in a vertical speed indicator is the pressure altitude rate of
change, since you fly pressure altitude.  So in this case ground
speed is accurate as well as the inertial vertical speed, but all
your information concerning the airspeed and wind speed is derived
in part from your static pressure.