August 5, 2014
By Barry Schiff
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:
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.
Barry Schiff has been an active certificated flight instructor since he was 18 years old.
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