It is called a vmu𝑣𝑚𝑢 (minimum unstick speed) test. The reasoning behind this test is to find the lowest speed at which an aircraft could take off, sustain controlled flight and safely come out of ground effect while maintaining full controllability. This vmu𝑣𝑚𝑢 is then used as a benchmark to calculate the various aircraft take off speeds. The rotation speed vr𝑣𝑟 and the lift off speed vlof𝑣𝑙𝑜𝑓 are calculated using corrections added to the vmu.𝑣𝑚𝑢. As vmu𝑣𝑚𝑢 is the absolute minimum speed at which the aircraft could lift off it can also provide data to the aircraft manufacturer to optimise the take off configuration to get the best possible performance out of the aircraft.
So, why the tailstrike during the take off? First off all not all the aircraft are subjected to a tailstrike during take off. This occurs only in aircraft that are Geometrically limited. By regulations, a geometrically limited aircraft should be in contact with the runway in the speed range of 96 to 100 of actual lift off speed of the aircraft. As this is not very practical, there is a leeway which states that the airplane needs to achieve this 50% of the time it is in the aforementioned speed range. The tail strike actually limits the angle of attack of the aircraft and prevents an overrotation by the pilot. This is an advantage as it puts the aircraft at less risk from an on ground stall. So, for geometrically limited airplanes the regulations allow a lower margin between vlof𝑣𝑙𝑜𝑓 and vmu𝑣𝑚𝑢. For non geometrically limited airplanes the vlof𝑣𝑙𝑜𝑓 cannot be less 1.05vmu𝑣𝑚𝑢(one engine inoperative) and for geometrically limited airplanes the margin is 1.04vmu.𝑣𝑚𝑢.
Airbus A340–600, the longest passenger aircraft undergoing vmu𝑣𝑚𝑢 testing.
It is usually those airplanes with shorter fuselages that do not fit in the geometrically limited criteria. Those pilots who have experience flying short versions of the otherwise the same aircraft will normally understand this. Ths shorter the aircraft, the more margin there is between the tail and the ground. This means, a very high pitch attitude will be required to get the tail planted on the ground, particularly on a take off. It is not an entirely safe thing to do either. So, in these type of airplanes the vmu𝑣𝑚𝑢 is reached by flying the aircraft at a high pitch attitude without hitting the tail on the runway.
Author – Anas Maaz
