Flight sample of the newest airliner on the runway. In the cockpit – an experienced crew of test pilots. The plane starts its run and … touches the runway with its tail. A sheaf of sparks is struck between metal and concrete. Spectacular, yes, but why spoil the plane?
Aircraft accidents with a “tail strike” (that is, with a tail hitting the runway) in aviation are not so rare, and one would think that by arranging such tests, aircraft manufacturers test their aircraft for strength. However, the reality is somewhat different.
Take off slowly
“The main goal of such tests,” says the honored test pilot of Russia Vladimir Biryukov, “is not to test the strength (although such a task is set), but to determine the so-called minimum aircraft take-off speed. For an airplane to take off, the lift force must be greater than the weight of the aircraft. The amount of lift depends on the area of the wing, the speed of the oncoming air flow (which is approximately equal to the takeoff speed) and the angle of attack of the wing. To get the minimum value of the speed at which the aircraft will take off from the ground, it is necessary to set the maximum pitch (attack) angle. The limiter of this angle during takeoff run is precisely the tail (the lower tail part of the fuselage). The tail touches the strip – the maximum angle has been reached. Typically, such – rather difficult and dangerous – tests are carried out at various values of thrust-to-weight ratio, that is, the ratio of thrust to the weight of the aircraft. The most difficult option is the one in which take-off with a maximum pitch angle is carried out at half the thrust of the engines.
Where are the sparks from?
Of course, the takeoff mode in which the tail touches the runway is extreme. When the airliner’s flight manual is ready, it will instruct line pilots to lift the aircraft off the ground at a speed much higher than the minimum established during flight tests. But in case of piloting error, for example, when the pilot takes the helm too early and abruptly, he will have some reserve, which will still allow him to take the plane into the sky. What is this reserve, just determined during the tests.
In fact, no one wants to spoil even a test flight model of a new airliner, and therefore, in test flights in which contact with the strip is expected, a special damper (heel, heel) is used, which is mounted on the tail of the aircraft. It is from him that a beautiful sheaf of sparks is cut. Naturally, the damper wears out gradually, although in general it is a fairly strong and wear-resistant device. The A350 minimum takeoff speed test report stated that three dampers were delivered to the airfield near Paris, where the tests were carried out, but only one cost.
Too soft, too hard…
And yet, blows by the tail of the aircraft on the runway happen outside of the tests. And then they can cause a serious accident. “Most often, the plane hits the runway with its tail during landing,” says Vladimir Biryukov. – The first possible reason is a landing at a speed lower than the calculated one. A special case of such a situation can be called the so-called long-term holding: in this case, the pilot tries to keep the aircraft in the air at a minimum height above the runway in order to “get used” to it with pneumatics and make a soft, impact-free landing. Naturally, in such a situation, the speed of the aircraft will gradually fall and, in order to maintain sufficient lift for the flight, the pilot will increase the pitch angle and lift the nose of the aircraft up. And this is fraught with touching the tail of the strip. Another reason is landing with excess vertical speed. In this case, when touched, the landing gear struts, designed for landing with an overload of not more than 2 g, are completely compressed. And then tail touching can happen. During takeoff, such cases occur much less frequently, but they also happen when, as a result of pilot error, the pilot takes the helm too early.
An example of an end-of-flight accident was eight years ago when an MD-81 on a charter flight from Copenhagen to Grenoble began an instrument approach in darkness. As a result, the aircraft landed on the runway with an excess of vertical speed and in a strongly pitching position (nose up high). There were 131 people on board, but, fortunately, no one was hurt, but the plane was seriously damaged. And everything could have ended much worse. On the net you can find a video in which a similar aircraft – MD-80 – simply loses its tail during a hard landing at an American air base.
Interestingly, tail strikes on the runway occur not only during the commercial operation of aircraft, but also during their testing, and in a situation where such a touch was not provided for in the test plan. Two years ago, a flight test sample of the latest Airbus A321neo was accidentally damaged in this way. Since the tests were “about something else”, no one thought to install a damper on the tail of the liner. As a result, the aircraft dropped out of the test program for several weeks and went into repair.
A blow by the tail of the aircraft on the runway is fraught with destruction of both the skin and the power structures of the liner, therefore, if something like this happens, the car will definitely be subjected to a serious technical inspection. And if such an inspection is not carried out carefully enough, a disaster can occur.
On August 12, 1985, the Boeing 747 of the Japanese airline JAL lost its vertical tail stabilizer (keel) shortly after takeoff, lost control and, after unsuccessful attempts by the crew to save the flight, crashed. The investigation showed that the roots of the tragedy go back to 1978, when the same plane hit its tail on the runway of the airport in Osaka during landing. Then not only the skin was damaged: the pressure bulkhead was damaged – a sealed bulkhead, a kind of plug that separates the leaky tail section from the passenger compartment, in which pressure is maintained at the level of the lower layers of the atmosphere. Damage to the pressure bulkhead was revealed, but the repair (which is surprising for the pedantic Japanese) was carried out poorly. For seven years, nothing foreshadowed trouble, but once the pressure bulkhead fell off under pressure, and a jet of compressed air sharply filled the cavity inside the stabilizer. It burst like an inflated paper bag. The liner was doomed.
Another Boeing 747, this time Chinese (China Airlines), waited for its bitter fate even longer. On February 7, 1980, the liner hit the runway with its tail, after which it was sent for repairs. Repairs were made, but again not too thoroughly. 22 years later, on May 25, 2002, the consequences of the strike made themselves felt. The liner, which was making a short flight between Hong Kong and Taipei, had its tail torn off right in the air, and the plane crashed into the waters of the Taiwan Strait, killing all passengers and crew.
“Of course, aircraft designers have to take into account the probability of tail hitting the runway as a result of piloting error,” says Vladimir Biryukov. “The part of the fuselage that will take the hit is reinforced with load-bearing structures that are more resistant to loads.” Also, in the most vulnerable place, engineers avoid placing electrical and pneumatic wiring, mechanical rods, which are control elements of aerodynamic rudders. The place of potential impact should not fall on the pressurized and inflated part of the fuselage.
Hitting the runway with your tail may not be the worst thing that can happen to an airplane. However, if such a situation has already been allowed, it is necessary to seriously assess the consequences of the shock for the structural elements and components of the liner. A small accident can one day turn into a big disaster.