Re: AA changes rules after BDL incident

From:         jas12@cus.cam.ac.uk (Julian Scarfe)
Organization: University of Cambridge
Date:         21 Dec 95 03:33:10 
References:   1 2
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> The plane should have been flying at 1,080 feet, but instead approached
> at less than 900 feet and sheared off the tops of oak trees along Metacomet
> Ridge in East Granby, Conn.

In article <airliners.1995.1955@ohare.Chicago.COM>, "G. E. Lambert"
<gedl@popd.ix.netcom.com> wrote:

> It is my understanding that AA flies all approaches, visual and
> instrument, using QFE altimeter settings.

If this is still the case and it comes out in the investigation that an
altimeter setting error was a contributory factor in the accident, it will
be very difficult for AA to continue to justify the use of QFE as a
landing datum.  AFAIR this is not the first AA accident that involved QFE
confusion.

In the UK, QFE is the default landing datum, though more and more pilots
and operators seem to be going over to the use of QNH.  Because UK
controllers are used to issuing QFE, the reminder to do this is a standard
part of radio procedure.  However, in a country where landing on QNH is
the norm, and QFE is presumably issued only on request or by arrangement
and it would be easier for the crew to use the wrong datum.

The danger is always that the crew forgets to switch from QNH to QFE at
the appropriate time (I have no idea at what point QFE is set in AA ops,
but it's typically somewhere between the IAF and FAF).  Inadvertently
using QNH in place of QFE would result in the altimeter overreading by the
airfield elevation.  With the usual disclaimers about speculation before
the publication of the NTSB report, it is interesting to note that BDL has
an elevation of 174 ft.  Thus the minimum descent height (above the
airport) for the approach was 908 ft, not much different from the
*altitude* (amsl) at which the CFIT took place.

> Does this mean that an aircraft flying a proper approach would have
> cleared the ridge by only 180 feet?  Isn't this too close for comfort?

I was going to say that I would email someone who might well know the
answer to this, but there seems to be no need.  He's beaten me to it:

http://www.terps.com/terps/bdl/welcome.html

...might be interesting reading for all those interested in this accident.

250 ft is the standard  required obstacle clearance (ROC).

Julian Scarfe
jas12@cus.cam.ac.uk