The devastation a bird strike can cause to an aircraft structure or engine is obvious.  But what about an insect? Or a few ice crystals? A surprisingly trivial substance can bring down even the largest aircraft.

All avionics systems rely on devices called pitot tubes.  These measure the force (impact) of oncoming air. The pitot-static system compares this impact pressure to the neutral (static) air pressure and feeds the information to three of the most critical instruments: Airspeed, Altitude and Rate of Climb. A blocked static port affects all three instruments, while the pitot tube itself will only involve airspeed.

The simplicity of the pitot tubes makes them vulnerable to blockages. The elaborite system of avionics, gauges and computers can be deceived by any number of blockages. Dead insects, mud wasp nests, and ice are the most common.

The most hideous aspect of this malfunction is the way a pilot, or even worse, computer, can be fooled. The flight instruments can provide wrong or conflicting information with a potentially tragic outcome. A pitot system failure is considered to be the worst possible malfunction of an aircrafts avionics.

Even a small amount of moisture can wreak havoc, as in the loss of a $1.4 billion stealth bomber on takeoff. This crash, the most expensive in the history of military aviation, was the result of not switching on the pitot heat system prior to air data calibration.  An X-31A crashed in 1995 after a pitot tube was blocked by ice. Many military aircraft are too unstable to control without automated avionics. In some situations the plane is not recoverable.

Austral Lineas Aereas Flight 2553 was lost, along with 74 lives, when a pitot tube froze solid. The DC-9’s instruments fooled the pilots into believing the plane was stalling. Pilots are trained to trust the instruments and computers, especially in darkness and poor weather. Spatial disorientation prevented the pilots from making proper decisions. Structural failure was the result of adding full power and lowering the flaps at high speed. Flight 2553 impacted the terrain at 1,200 kph.

In 1996, Birgennair Flight 301 crashed shortly after takeoff. Mud wasps built a nest in one of the Boeing 757’s pitot tubes. The captains airspeed indicator showed 350 knots and triggered the autopilot to pull up and reduce power. The actual airspeed was much slower. The pilots heard contradictory overspeed and stall warnings. Although the co-pilots separate pitot system was working, they could not know which to believe. The stall warning was very real but was ignored to within 8 seconds of the ground. 183 people were killed. Ground crews could have saved Flight 301 by installing pitot tube covers while it sat idle.

That same year Aero Peru Flight 603 was lost because ground crews failed to remove a piece of duct tape from the static ports on while cleaning a 757. The computers issued over speed, under speed and “too low, terrain” warnings. Again, the pilots had no way of knowing which warnings were correct.  This crash illustrates just how deadly a failed pitot-static system can be. The simple devices are the source of the entire system. The failure of even one of the two or three sensors can circumvent the whole concept of redundancy. Besides feeding information to the instruments and computers, the faulty readings were also relayed to the air traffic controllers. The aircrafts computer fooled the controllers into confirming horrifically wrong speed and altitude data.   With this information they confidently flew the aircraft into the ground. All 70 on board were killed.

It’s hard to believe how many aircraft and lives have been lost to such trivial events.  It’s unsettling to realize they will not be the last.