I’m sure you’ve seen video of a Boeing 737 lifting off as yet another news reader drones on about the MCAS troubles in the MAX version of the world’s most popular airliner. If you watched closely, you have seen what looks like a wire or tube with a cone on the end trailing from the top of the rudder.
What the heck is that thing, and why is the 737 dragging it through the air?
The video is of the 737 in test configuration and the tube and cone are a static pressure port. The lash-up is usually called a “static cone.”
One of the first, and critical tasks, in any flight test program is to calibrate the airspeed and altimeter systems. Especially airspeed. If you don’t have reliable airspeed data, then you can’t know for sure how the airplane is performing, at what airspeed it will stall, and at what high airspeed flutter and other structural issues threaten.
All flight test data, and thus all data essential for safety and certification of any airplane, depends on reliable airspeed information. If you don’t know how fast the airplane is flying, you really don’t have any useful information.
We all remember from private pilot ground school that indicated airspeed is the difference in the ram air pressure entering the pitot tube and the static pressure of the atmosphere surrounding the airplane. Sounds easy, right? And on smaller airplanes flying at low airspeeds, measuring pitot and static pressure reliably may not be too difficult.
But on larger airplanes, flying at a wide range of airspeeds from takeoff, landing, to near the speed of sound in cruise, the challenge of measuring pitot and static pressure can be very complicated.
Of the two inputs, measuring pitot pressure is almost always easier. On most early test flights of a new or modified airplane, you see a long boom protruding from the nose. That “test boom” holds a pitot tube that is located forward of the airplane and is mostly free of the air flow distortions created when the slip stream accelerates to flow around the airplane.
Airplanes create a “bow wave” when they move through the air similar to a boat pushing a wave ahead and around it as it moves through the water. The test boom moves the pitot tube ahead of the bow wave, or at least ahead of its greatest disturbance.
The entire airplane is also creating a wake as air moves out of the way to pass over the airframe. That wake, or movement of air, means the air is not “static” around the airplane. But we need “static” air to complete the pitot-static airspeed measurement. That’s where the static cone comes in.
By trailing a static port far behind the airplane, the static port remains clear of most of the air pressure disturbance caused by air flowing around the airplane. Often the static cone and its tube are on a reel that the crew can extend after takeoff to trail the cone farther behind in “clean” air after takeoff. Sometimes the test pilots reel out the cone by taxiing slowly ahead on the runway before beginning the takeoff roll.
Advanced fluid dynamic programs do a good job of predicting how air flow behaves over all parts of an airplane. Using those programs designers find locations on the airplane where airflow is most stable across the performance envelope and that’s where they plan to mount the actual static ports and pitot tubes.
Flying with the pitot test boom and the static cone provides quite precise airspeed indications that can be compared to the actual static ports mounted somewhere on the airplane, which is most often the forward fuselage on fast airplanes, and tailcone of slower airplanes.
Test pilots also typically use a “chase” airplane to confirm pitot-static performance, particularly at very high airspeeds. The chase airplane has a well established airspeed indication system and so by flying it alongside the test airplane the pitot-static system of the new airplane can be confirmed.
Development of the digital electronic air data computers most of us now fly with – even in basic piston singles with flat panel integrated avionics – makes it possible to correct for pitot-static errors. Those errors can be caused by configuration change, such as extending the flaps, or flying at higher or lower altitudes, and faster or slower airspeeds. But the errors must first be documented in flight test and that’s why the static cone is so essential. The air data computers are terrific at correcting errors, but only after the errors themselves have been documented.