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.
Web: www.barryschiff.com
Barry Schiff has been instructing for 58 years and has seven flight instructor ratings.
