AOPA Pilot Magazine - October 2014 - 16

PROFICIENT PILOT

Gotta have static

BY BARRY SCHIFF

The trouble with blocked systems

BARRY SCHIFF

has been
instructing for
58 years and has
seven flight
instructor ratings.

16 | AOPA PILOT October 2014

ON THE EVENING OF OCTOBER 1, 1996, maintenance personnel at the international airport serving Lima, Peru,
were tasked with washing an AeroPerú Boeing 757
scheduled for a late-night departure to Santiago, Chile.
Prior to cleaning the airplane, the static ports had been
covered with duct tape to prevent contaminants from
entering the static system (a good thing). Unfortunately,
the tape was not removed after the cleaning had been
completed (a bad thing). Nor did the pilot charged with
conducting the preflight inspection take sufficient care
to notice that the static ports were blocked.
The aircraft departed over the ocean shortly after
midnight. Minutes later the crew reported having difficulty with their flight instruments and wanted to return
for landing. Following several moments of crew confusion-the natural horizon was not visible-and various
cockpit warnings, the aircraft plummeted into the
Pacific blackness and killed all 70 on board.
Last month in this column I discussed how I had
used Scotch tape to block the entry hole of a pitot tube to
determine if my students would notice this during preflight inspection ("Proficient Pilot: Pitot-Static Pitfalls,"
September 2014 AOPA Pilot). Most did. Not as many,
however, detected when I committed similar sabotage
to the static system. This likely is because static ports
are less conspicuous, and we don't always inspect them
as carefully as we should. Although blocked static ports
are rare, it can happen. Static sources are susceptible to
bugs, dirt, wax, blowing sand, and so forth. Water in the
lines also can create problems-especially if it freezes.
A pilot taking off in a lightplane with blocked static
ports will know it soon enough. Most obvious is that
the altimeter and vertical-speed indicator will not operate during climb; they will continue to insist that the
aircraft had never left the ground. Interestingly, the airspeed indicator will operate normally throughout the
takeoff roll and seem to operate normally during initial
climb. As the aircraft gains altitude, however, indicated
airspeed will become increasingly less than it should be.
In other words, a pilot will be led to believe that the airplane is flying more slowly than it really is. The opposite
occurs during descent.
Many pilots believe that (most) airplanes have two
static ports to provide for system redundancy. Not so.
Otherwise, such considerate manufacturers also would
provide a backup pitot tube because it is more prone to
icing and contamination.

Dual static sources are provided to balance pressure
variations that occur on opposite sides of the fuselage
during slips and skips, and when in turbulence.
For example, assume that that some sadistic flight
instructor-I deny ever having done this myself-covered the right static source with Scotch tape before
departure. This would have no effect during smooth,
coordinated flight. Everything would appear normal
until the airplane is made to slip or skid. For example, if
the pilot were to alternately move the left and right rudder pedals, the instruments would behave erratically.
Why they would do so is not mysterious. During a right
yaw, the only open static source (on the left) would be
turned partially into the relative wind. The airstream
would blow into and somewhat pressurize the left static
port. This increased static pressure would cause the
altimeter to show a loss of altitude and the VSI to indicate a sink rate. Indicated airspeed also would decrease.
Conversely, a left yaw would result in the open static
source to be on the "downwind" side of the fuselage.
This would result in a decrease in sensed static pressure. Indicated altitude would increase, the VSI would
"climb," and indicated airspeed would increase.
In turbulence, the three pitot-static instruments
would be very sensitive and fluctuate rapidly between
high and low indications.
If a pilot suspects a problem with his static system, he should immediately use the alternate static
source (although not all airplanes have one). This is an
extension of the main static line that is routed into the
cockpit and installed within easy reach of the pilot. The
tube is sealed with a petcock so that cockpit air cannot normally enter the static system. In the event of a
static-system anomaly, however, opening the petcock
introduces ambient cockpit air to the static instruments.
Since the air pressure in an unpressurized cockpit is
nearly the same as ambient pressure outside the aircraft,
this restores reasonable instrument accuracy.
Static systems also can become partially clogged.
This can be particularly confusing because the symptoms are not as easy to diagnose and vary according to
the degree of blockage. Be equally vigilant about checking the fuel-tank vent. Departing with a clogged vent
can result in fuel starvation, a collapsed fuel tank, and
possibly a collapsed wing.
AOPA
WEB

www.barryschiff.com


http://www.barryschiff.com

AOPA Pilot Magazine - October 2014

Table of Contents for the Digital Edition of AOPA Pilot Magazine - October 2014

Contents
AOPA Pilot Magazine - October 2014 - Cover1
AOPA Pilot Magazine - October 2014 - Cover2
AOPA Pilot Magazine - October 2014 - Contents
AOPA Pilot Magazine - October 2014 - 2
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AOPA Pilot Magazine - October 2014 - R-1
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AOPA Pilot Magazine - October 2014 - 65
AOPA Pilot Magazine - October 2014 - 65a
AOPA Pilot Magazine - October 2014 - 65b
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AOPA Pilot Magazine - October 2014 - Cover3
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