Re: Continental 727 nearly belly-flops at O'Hare

From:         rdd@cactus.org (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         04 Jan 94 22:52:23 PST
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In article <airliners.1993.812@ohare.Chicago.COM> mmr47784@uxa.cso.uiuc.edu (Mark Rogers        ) writes:
>Normal airline procedure is to respond to the GPWS, THEN determine what
>caused it.  

I've never heard of this as a procedure.  The only system which really 
requires "blind faith" responses (courtesy FAA/company policies) is TCAS.  
GPWS is notoriously unreliable, and warnings can be rather vague (whoop whoop 
pull up for four of the five modes, in older systems).  Therefore, the normal 
response to this *caution* is to reconcile the warning with one's situational 
awareness.

In this case, the crew was in visual contact with the runway when they got the 
GPWS warnings.  It is not clear whether they were using an old system or a 
new(er) system.  

I'd probably have ignored the GPWS too, if I was obviously on glide slope and 
had the runway in sight.  Three sets of eyes ignoring three bright green 
lights on the gear indicator is a bit harder to rationalize. :-) Warning or 
not, what happened was *not* a consequence of GPWS being disabled or ignored, 
but rather the broader airmanship issues of the approach.  


>The five reasons, and the radio altitudes they occur at are:
>
>      Altitude                                Reason
>
>      <2450                          Excessive Barometric Descent Rate
>      <1800                          Excessive Terrain Closure
>      <500                           Gear Handle not Down
>      <200                           Landing Flaps not Selected
>
>(The fifth reason would be after a go around, and less than 700 feet, when
>a descent of 10% of altitude occurs)

A Sundstrand implementation I'm familiar with, and which is installed in at 
least the 737, 747, and some 727s, is merely classified by the category of the 
warning.  Each category generally has an envelope which is defined by at 
least two slope curves, and two of the categories have two engage thresholds, 
depending upon configuration.  The actual warning depend upon which part
of the curve one is in (i.e., faster whoop-whoops when you're in greater
peril).


>From what I have read, this was one of the flight crews big mistakes- The
>flaps were left at 25 to keep the speed UP.  Flaps 25 is not a legal landing
>flaps setting in the 727- that's why they didn't get a horn.  If they had
>extended the flaps to 30 (a legal setting), they would have heard an 
>unsilencable horn as the flaps passed through something like 27.5 degrees.
>In any event, the crew was violating regs if they were trying to land with
>Flaps 25.  

I am not convinced of that.  The 727 is certified for flaps-up landings, even
though there are no charts for that as part of normal ops manuals: the only 
thing missing at flaps 25 is that they wouldn't have the performance info 
(well, maybe--it might be in a PDCS, if installed).  Let's classify this as 
a "gray area."  Perhaps the crew felt they could safely land the airplane 
on the 10000' dry runway available to them.  Given the ATC constraints, 
FAR 91.3!







---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd