A few weeks back, we heard the news of an Ethiopian Airlines Boeing 787 suffering from a tail strike during landing.
A few weeks back, we heard the news of an Ethiopian Airlines Boeing 787 suffering from a tail strike during landing. And just at the beginning of this year, a British Airways Airbus A350 had a similar event.
The Ethiopian case is still under investigation. However, the British Airways case has already been investigated and the reason behind the incident was attributed to pilot error. And this is a common finding in almost all tail strike events that have occurred in the past - they are caused by the human component.
One of the most interesting things in these two events is that the Airbus A350 and the Boeing 787 are the most advanced aircraft in the skies today, with so many inbuilt protections, including systems to prevent tail strikes. This shows the important role the human pilot plays in keeping the tail of the aircraft safe during take-offs and landings.
A tail strike is an event whereby an aircraft tail contacts the runway. Tail strikes occur during take-offs and landings. When it occurs, it can cause significant amounts of damage to the aircraft, sometimes requiring major repair work.
The longer the aircraft is, the more prone it is to a tail strike. When the aircraft is long, it is said to be geometrically limited, and this can lead to lower tail strike margins.
The data and statistics show that most tail strikes occur during landing. According to Airbus statistical data, over 65% of tail strikes happen during landings, while only 25% occur during take-offs.
There are several reasons why a tail strike might occur on take-off. In the following paragraphs, we will look at some of them in more detail.
The aircraft is designed to lift off at a particular speed. When below this speed, the aircraft wings cannot produce sufficient lift to fly off the runway. During the test phase of a prototype aircraft, a test called Vmu (Minimum unstick speed) test is performed. This test is performed to find the lowest speed at which the aircraft could safely lift off.
As this test is carried out at a very low speed, in a long aircraft, this leads to a tail strike. And for testing purposes, this is quite normal. In such a situation, the wings do not provide the required lift, but the thrust from the engines generates a vertical vector that pushes the nose of the aircraft upwards. The vertical attitude of the aircraft is then limited by the tail hitting the runway. The tail is then dragged on the runway until the wings generate enough lift to finally lift the aircraft off the ground.
By doing this test, the manufacturer can determine the very basic performance of their prototype and use the speeds and data gained from the test to come up with operational speeds such as the rotation speed (Vr), which the normal pilots use to initiate the rotation for lift off. This Vr speed is way above the Vmu.
When flown at Vr, the aircraft tail will stay well clear. But then, why do tail strikes occur? One reason is a miscalculation of the Vr speed during pre-flight. The speed for Vr is directly proportional to the weight of the aircraft. The heavier it is, the higher the Vr. Thus, if the pilot inputs a lower Vr than the actual Vr and rotates at the wrong, lower Vr, the aircraft might suffer from a tail strike.
The rotation for lift-off is one single maneuver, whereby at Vr speed, the pilot must pull back on the stick at a pitch rate of about 3 degrees per second. In heavier, longer aircraft, due to inertia, the aircraft can sometimes react slower to pilot actions on the control stick or yoke. The worst thing a pilot can do at this point is to pull back more on the stick to increase the pitch rate. This will add to the already developing pitch rate and make the rotation rate too quick, making the tail strike the runway.
Pilots who transition from smaller aircraft to much larger aircraft must be very aware of the proper technique for rotation. In smaller aircraft, with manual (cable) powered controls, sometimes a lot of effort is required from the pilot during the rotation. In aircraft with hydraulic controls, being aggressive during rotation could cause a build-up of excessive pitch rate, which could, in certain cases, cause a tail strike.
The aircraft's CG location determines the handling of the aircraft. The more aft or back it is, the more sensitive the aircraft is in pitch control and the more forward it is, the heavier the aircraft is in pitch. The latter rarely leads to tail strikes as it prevents an overcontrol situation.
However, when the CG is well back, the ever-sensitive pitch control may lead to overcontrol, and the tail may contact the ground.
During pre-flight, the pilots set the stabilizer of the aircraft to ensure that the aircraft is trimmed to the correct setting. When the aircraft is, for instance, loaded in such a way the nose is heavy, the trim is set such that the effort the pilot must apply on the controls is reduced so that the aircraft lifts off the runway easily.
If the pilots were to mistakenly set the set stabilizer trim more nose up than is required, then the aircraft might be a little over-sensitive in pitch, and this may lead to a tail strike.
Research has shown that tail strikes that occur during landing cause more damage to the aircraft. One of the reasons behind this is the fact that during landing, the tail may strike the ground before the main landing gear making the aft pressure bulkhead absorb all the energy. Also, during the landing, the aircraft is in a low energy state, and this can make the tail drag on the runway for a longer period.
As for the take-off, many factors lead to a tail strike on landing.
The major contributor to tail strikes on landings is unstable approaches. A stable approach is one where, the aircraft is at the correct speed, and correct glide while tracking the runway center line. Below, we are going to look at things that make the approach unstable and how they may lead to a tail strike.
The aircraft approach speed must be maintained until the point of touch down. A decrease in speed causes a decrease in lift and aircraft energy. It is this energy and lift that cushions the landing when the pilots eventually pull back on the controls for the landing flare. In the absence of this, the only way to reduce the sink during the touchdown is to pull on the controls harder, which increases the pitch attitude of the aircraft. This reduces the clearance between the tail of the aircraft and the ground.
Aircraft should be flared in accordance with its size and how inertia works on it. Generally, heavier and larger aircraft require an earlier flare than smaller aircraft.
When the aircraft is flared by pulling back on the controls earlier, the aircraft tends to start to lose speed and float, and when the pilot pulls on the controls further back as the aircraft approaches the touch-down point, there is an increase in pitch attitude and a greater loss of speed. This can result in a tail strike.
Similarly, when the aircraft is flared too late, the pilot may tend to flare more by giving larger pull-back inputs. This can, again, increase the pitch attitude of the aircraft resulting in the tail scraping the runway.
During crosswinds, the aircraft needs to be cross controlled at the touch-down to minimize the lateral forces on the main landing gear. Initially, the aircraft approaches with a crab, where its nose is pointed into the wind, while the track of the aircraft follows the runway center line. When cross controlled, the rudder is applied to center the aircraft to the runway center line while the roll controls are applied to keep it from drifting.
When roll controls are applied, in most large aircraft, the roll spoilers also come up, which reduces the overall lift on the wings. The cross-controlled situation also increases the drag on the airframe, which leads to a high sink rate. To correct this sink, the pilot may pull back on the controls to the point the tail strikes the ground.
When a go-around is performed close to the ground, particularly with engines in idle power or thrust, the tail may strike. One of the main reasons for this is the fact when the thrust levers are moved forward to add thrust for the go-around, it takes about 7-8 seconds for the engines to spool up. If the pilot pulls back too much to get the aircraft flying for the go-around, with engines not spooled up, the lack of aircraft energy could result in a tail strike.
This was what happened in the recent British Airways Airbus A350 tail strike incident.
When taking off, tail strikes can be reduced by adhering to the correct procedures. The pilots must ensure that the take-off speeds are correct. This can be done by two pilots independently checking and cross-checking the performance that is entered into the aircraft flight management system. This way, any inconsistent data, such as an incorrect Vr speed, may be found before the take-off is commenced.
The rotation during the take-off must be a calm continuous maneuver to achieve a 3-degree per-second rotation. There is no point in being too aggressive during the rotation, particularly in a longer aircraft. When transitioning to a new model of an aircraft, the pilots must be aware that the new aircraft may not have the same characteristics as his or her previous aircraft.
To make sure that the aircraft is loaded properly, the pilot must check the final load sheet and the trim setting. If he or she feels that the trim value in the load sheet is not appropriate, they must raise questions with the load control as the captain of the aircraft has the final say on how the aircraft is to be loaded.
It is also important to ensure that the physical trim on the aircraft is set properly during pre-flight. Even after all these checks, the aircraft may still behave incorrectly during the takeoff because the pilots see the numbers and not how the actual loading was performed. So, an incorrect loading might still give the aircraft undesirable rotation characteristics.
If the aircraft is found to be too pitch-sensitive during rotation, the pilot must try not to play it with too much and get it airborne. These things should not be much trouble for a well-trained pilot. Once on the ground after the flight, the incident should be reported to the airline’s safety department for a proper investigation.
The most important thing on landing is to always be stabilized. Unstabilized approaches put the pilot under stress to get the aircraft stabilized, requiring large control and thrust lever movements. The aircraft should always be at the correct speed and trajectory during the final approach to landing.
The flare for the touchdown should be initiated at the correct point as applicable to the aircraft. It should not be too low or too high. A stabilized approach ensures that the aircraft is on a correct trajectory at flare height.
During a low-altitude go-around, the pilots must be aware of the engine spool-up time and must not be too quick in increasing the pitch attitude of the aircraft. In larger aircraft, pilots should expect wheel contact with the ground in a low go-around. This is normal, as the inertia of the aircraft does not allow it to fly away that easily. So, when such a contact is made, the pilots should not alarm themselves and pitch up more. This could lead to a tail strike.
Modern aircraft, particularly those with fly-by-wire control systems, have tail protection systems.
In takeoffs, what the pilot feels in his hands plays a major role in how he initiates the rotation for takeoff. This “feel” varies with aircraft weight and CG location, thus ensuring that the feel to the pilot is the same at all CGs and weights to reduce rotation inconsistencies. In those aircraft with hydraulically actuated controls (all heavy aircraft), the artificial feel system can be tweaked to give the pilot a similar feel for rotation. This is a feature found in many aircraft with hydraulically operated flight control surfaces.
In fly-by-wire aircraft, tail strike protection systems are built and written into the control software. In the Boeing 787, the Primary Flight Computers (PFCs) monitor the tail clearance at all times. If this clearance goes below a threshold, the elevator deflection is automatically reduced without input from the pilot. This reduces the chance of a tail strike if the pilot were to mishandle the aircraft.
In older Airbus aircraft, the rotation on takeoffs is quite conventional. The rotation rate during take-off, for example, varies for a given stick input based on aircraft weight and CG. Even in these aircraft, however, the maximum elevator deflection angle is reduced during take-off to prevent overcontrol.
In newer Airbus aircraft, the rotation is homogenized. That is, the input on the side stick is now converted to a pitch rate demand which is fixed and does not vary with outside and aircraft conditions. Like the Boeing system, the aircraft computers at all times monitor the tail clearance with data from the radio altimeters and use it to vary the elevator deflection to prevent tail strikes both on take-off and landing.
Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.