Break the VSI?

Date:         21 Nov 96 03:02:19 
From: (MiDiBu)
Organization: MindSpring Enterprises, Inc.
References:   1 2
Followups:    1 2
Next article
View raw article
  or MIME structure

Tom Stybr <> wrote:

>It has been reported that a maintenance crew covered the static ports
>with duct tape during a skin polishing task and then neglected to remove
>it. The pilot also failed to notice the covered ports during his
>walk-around (if he did one).

>This explains the pilot not knowing his airspeed or altitude. The static
>lines would have had higher than actual static pressure causing
>instruments to read lower altitude than actual (his ground prox alarm
>was sounding). The airspeed instruments use both static ports and pitot
>probes to determine airspeed.

>Dense fog contributed to no visual checks of the instrument readings,
>the pilot became disoriented, you know the rest.

Yes that is true.  But what I'm wondering is that, from what I've
heard, the pilots told ATC that they had a problem.  They tried, I
guess, use the radio altimeter to get their altitude.  The ATC told
them that they were at  (approx) 9600 feet and to descend.  They did
and skipped off of the water, and cartwheeled.

One of the oldest tricks that a pilot learns is that if the static
ports are clogged up, one breaks the glass on the vertical speed
indicator (VSI).  Before you see the problem with this I would like to
address two things:

1.  The cabin was pressurized.  OK, depressurize it.  At near sea
level it would cause no harm.

2.  The plane had a glass cockpit (ie, no VSI to break).  Well, that
may be true ( I really don't think that smashing a fire axe into a CRT
would help the air data computer one bit).

However, in all of the planes that I've seen, there are three little
standby instruments on the panel, between the pilots, that give the
most vital information.  There is a "JET" attitude indicator. There is
an airspeed indicator.  And there is an altimeter.  They are there
purposely to give the pilots a last chance ability to fly the
airplane.  The "JET" is powered by a self-contained battery.  The
other two, as I have assumed up to this point, are pitot/static
instruments that have been in use since the 1930's.  I think that the
"JET" even has a slip/skid ball.
Given that one can fly an airplane on "needle, ball, airspeed, and
altimeter", would it not be possible to crack the glass on either the
altimeter, or the airspeed indicator, and at least have a reasonable
idea of the the actual configuration (not exactly the word that I'm
looking for)  at least to the precision to determine that you were
indeed closer to 9600 feet, rather than sea level.
I don't want to dwell on this, but I did take off one time with the
statics ports on my Mooney clogged up because I had waxed the airplane
before the flight.  It was a short runway, acceleration seemed good,
so I just flew it when the airpeed only read zero.  A perfect VFR day
and it was no big deal.  And, no, I didn't break the VSI face glass.

But, the reason that I'm posting this is - Could the pilots of a 757
break the glass on the pitot/static instruments and undo the damage
done by the ground crew/bad walkaround?

Mike Weller