It goes almost unnoticed, yet when one looks more closely, it’s impossible not to see this cord and shuttlecock hanging from the top of the tail fin. It is in fact, the “Trailing Cone”. Present on all development aircraft during the first weeks of flight tests. The nylon cable is deployed in flight, generally 1 to 1.5 times the wing span length, or about 50 metres, behind the aircraft to measure the ambient atmospheric pressure (static pressure) precisely, in the “free-stream air-flow” outside the disrupted air-flow generated by the aircraft - a key parameter for the pilots.
As Pierre Baud, former flight test director at Airbus, explains: “the static pressure is in fact absolutely necessary to measure the plane’s exact airspeed and altitude. The static pressure is therefore fundamental to measure the performance of a test aircraft.’’
In tests yes... Because once the aircraft is in service there’s no reason to have a cable-trailing behind. To measure the static pressure, commercial aircraft are in fact fitted with sensors on the fuselage; sensors that are therefore subject to disturbed air-flow and thus giving inaccurate data. That’s the reason why, in order to correct these calculations, the trailing cone is used during the developmental phase.
The test engineers determine the disrupting effect of the perturbed airflow of the plane with the sensors that are on the aircraft fuselage in relation to specific measurements that are done outside this disturbed flow. These data are then put into a computer and the resulting corrections from the flight tests are applied.
Once this margin of error has been calculated, and the correction rate found, the cable can then disappear. Strictly speaking the cone plays only a stabilizing role; but sometimes, in the past, has finished up in the treetops. The test engineers on board having simply forgotten to wind the cable back in before landing. For test technicians that’s called "giving the cable a hard time." Or could we say, sometimes, there’s a twist in the tail?