AOPA Pilot Magazine - September 2014 - 16

PROFICIENT PILOT

Pitot-static pitfalls

BY BARRY SCHIFF

The dangers of a clogged pitot tube

BARRY SCHIFF

has been an active
certificated flight
instructor since he
was 18 years old.

ONE OF MY favorite instructional aids is a roll of Scotch
tape. At some point during a pilot's training (advanced
or otherwise), I might use a piece of said tape to cover
either the opening of the pitot tube or one (or both)
static sources.
It is not easy to detect a piece of Scotch tape during a preflight inspection, and this seemingly sadistic
technique provides a good opportunity for me to determine how thoroughly a pilot checks his airplane. This
also opens the door to a discussion about the consequences of operating with a partially or totally disabled
pitot-static system.
(As a young instructor, I occasionally allowed a
student to depart on our dual flight with a sabotaged
system that he had failed to discover. This obviously was
illegal because the airplane was not airworthy. Much
older and minimally wiser, I would not do that again. I
admit this now because of my presumption that the statute of limitations has long ago expired and to discourage
other instructors from being tempted to do the same.)
A spectacular example of an accident that can
result from a clogged pitot tube occurred in 1974 when
the pilots of a Northwest Airlines Boeing 727 departed
JFK for Buffalo. Although cleared to Flight Level 310,
the ill-fated flight never got there. At 24,800 feet, the
Boeing entered a spin and crashed only 83 seconds
later (after descending at an average of 17,000 fpm).
Information from the voice and flight-data recorders recovered from the wreckage led to the inescapable
conclusion that the pilots had failed to turn on the
727's pitot heaters as required by the Taxi Checklist.
As a result, a lethal dose of icing clogged the pitot tubes
during climbout.
The Boeing pilots were then misled by erroneous airspeed indications. This is because clogged pitot tubes
cause their associated airspeed indicators to behave
like altimeters and respond only to changes in altitude.
During the climb, indicated airspeed in the 727 increased
steadily and persistently. This caused the pilots to raise
the nose farther in an attempt to arrest what appeared
to be dangerously increasing airspeed. This resulted in
an increased climb rate, which compounded the problem. As altitude continued to increase, so did indicated
airspeed. The pilots continued raising the nose until they
had unwittingly forced the aircraft to stall.
There are three lessons to be learned from this
accident:

16 | AOPA PILOT September 2014

1. Adhere diligently to checklists.
2. When instruments disagree, initially believe
the one indicating the most adverse condition. (Both
attitude indicators in the 727 operated properly and
reflected a 30-degree pitch angle prior to stall.)
3. Distrust instruments that indicate performance
beyond the capability of the aircraft. (Near the top of
the 727's climb, the VSI indicated 5,000 fpm, and indicated airspeed was 340 knots, an impossible feat.)
If the pitot tubes had instead become clogged by
ice during descent, the airspeed indicators still would
have behaved like altimeters. Indicated airspeed, however, would decrease, tempting the pilots to push the
nose down in a misguided attempt to arrest decreasing
airspeed. Actual airspeed would then become dangerously higher than indicated.
During cruise flight, clogged pitot tubes might not
become immediately noticeable because indicated airspeed remains more or less the same as it was before
the clogging occurred.
One does not have to be in icing conditions to experience a clogged pitot tube. This can occur when the
pitot tube impales or inhales an insect. On the ground,
it can be caused by a bug's nest, wax, mud-or a runaway instructor with a roll of Scotch tape.
An interesting situation occurs when there is a
drain hole in the pitot-tube mast that allows water to
drain from the system. If such a pitot tube becomes
clogged with ice or for other reasons, indicated airspeed drops to zero because trapped ram-air pressure
bleeds out of the drain hole. It pays to know, therefore,
if the pitot tube on the aircraft you fly has a drain hole,
because pitot-tube clogging is manifested differently
than when the tube does not have one.
An excellent way to prepare for the possibility of
an airspeed indicator failure is to be intimately familiar with the equation, "pitch + power = performance."
This is a simplified way of saying that aircraft performance depends on power setting and pitch attitude.
If a pilot knows the power and pitch normally associated with an approach to landing, he can safely make
that approach without an airspeed indicator. It is a skill
that all pilots should have.
Clogged static ports and lines cause a different set
of challenges. We'll visit these next month.
AOPA
WEB

www.barryschiff.com

http://www.barryschiff.com

AOPA Pilot Magazine - September 2014

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