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|Category:||Loss of Control|
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A tail strike occurs when the aft fuselage (tail) of an aeroplane comes in contact with the runway during either takeoff or landing. Statistically, the majority of tail strikes occur on landing. Tail strikes most often occur as a result of human error although environmental factors, such as strong gusty winds, can increase the potential for an event.
Tail strike, which occurs when the tail of an aircraft contacts the runway during takeoff or landing, is an event that can occur in virtually all transport aeroplane designs. Some designs are far more susceptible to tail strike than others and, dependent upon the aircraft type and model, the relative frequency of tail strike on takeoff versus tail strike on landing can vary significantly. "Stretched" models of a given type are generally more likely to suffer a tail strike than the non-stretched version.
Various studies by several of the major aircraft manufacturers have arrived at similar conclusions regarding the primary cause of tail strike. Although the event has occurred during both daylight and night operations, and in both good weather and bad, the most significant common factor has been found to be the amount of flight crew experience with the specific model of aircraft being flown. This conclusion points to robust crew training as the single most critical preventative measure.
Studies have also identified eight specific Causal Factors, evenly distributed between the takeoff and landing phases of flight, which greatly increase the risk of a tail strike. Note that combinations of these Causal Factors can occur, further increasing the likelihood of a tail strike event.
One or more of the following four Causal Factors have been identified as causal in the majority of tail strike events during take off:
- Improperly Set Elevator Trim or Mis-Trimmed Stabiliser - A mis-trimmed stabiliser or mis-set elevator trim during takeoff is not a common occurrence. When it does occur, however, it is usually as a result of mistakes in the Load and Trim calculations leading to erroneous data, the wrong weights or an incorrect centre of gravity (CG). There have also been cases where the information presented to the flight crew was accurate, but it was entered incorrectly either into the Flight Management System (FMS) or to the stabiliser itself. In either case, the stabiliser is set in the wrong position. When entering data into the FMS it is always good to do a quick ‘Reality’ or ‘Gross Error’ check. As Davis wrote in ‘Handling the Big Jets’ it is always good practice to have clear idea of the approximate weight of the airplane in your head at all stages of flight. (The number of Pax x 100 less 10% can be a quick way to check that part of the load.) If the control surface is mis-trimmed nose down, it can present several problems during the takeoff but tail strike usually is not one of them. However, if the surface is mis-trimmed in a nose up direction, it can place the tail at risk. This is because the yoke now requires less pull force to initiate aircraft rotation during takeoff and the pilot flying (PF) may be surprised at how rapidly the nose comes up. Most manufacturers recommend a rotation rate between 2.0 and 3.0 degrees per second and liftoff in most large aircraft will occur 3 to 4 seconds after the nose begins to rise. However, if the stabiliser has been mis-trimmed, the nose can rise very rapidly reaching the angle at which tail strike will occur before there has been time for the aircraft to become airborne.
- Rotation at Incorrect Speed - This situation can result in a tail strike and is usually caused by a VR that has been computed incorrectly and is too low for the weight and flap setting. In the Airbus A340-500 incident cited below, a weight of 100 tonnes less than the actual weight was loaded into the FMS leading to erroneous speeds and thrust setting which, in turn, resulted in a tail strike and runway overrun. This further reinforces the importance of pilots cross-checking and gross-error-checking all FMS inputs. Tail strike, due to early rotation, can also occur if the PF initiates the rotation at the "V1" call, or at any other time prior to the "rotate" call, instead of at the "rotate" call.
- Excessive Rotation Rate - Pilots operating an aeroplane type that is new to them, especially when they are transitioning from unpowered flight controls to ones with hydraulic assistance, are most vulnerable to using an excessive rotation rate. The amount of control input required to achieve the correct rotation rate varies from one aircraft type to another. When transitioning to a new aircraft, pilots may not consciously realise that it will not respond to a pitch input in exactly the same way as their previous aircraft type. On some aircraft types, this pitch response differential is particularly important when the CG is loaded toward its aft limits. This is because an aeroplane in this configuration is more sensitive in pitch, especially during takeoff. A normal amount of nose up elevator input in an aft CG condition is likely to cause the nose to lift off the runway more rapidly and put the tail at risk.
- Improper Use of the Flight Director - The flight director (FD) is designed to provide accurate pitch guidance only after the airplane is airborne. An aggressive rotation into the pitch bar at takeoff is not appropriate and may actually rotate the tail onto the ground. Best practice is to rotate to an Attitude.
One or more of the following four Causal Factors have been identified as causal in the majority of tail strike events during landing:
- Unstabilised Approach - An unstabilised approach, in one form or another, is a factor in virtually every landing tail strike event. Flight Data Recorder (FDR) data demonstrates that flight crews who continue an unstabilised approach below 500' AGL will likely never get the approach stabilised. In the flare, the aircraft invariably will have either excessive or insufficient airspeed and quite often will also be beyond the runway touchdown zone. The result is a tendency toward large power and pitch corrections in the flare, often culminating in a substantial nose up pull at touchdown followed by a tail strike. If the nose is coming up rapidly when touchdown occurs and the ground spoilers deploy, the spoilers themselves add an additional nose up pitching force. Also, if the airplane is slow, pulling up the nose in the flare does not materially reduce the sink rate and in fact may increase it.
- Excessive Hold Off in the Flare - A common cause of a tail strike during landing is a long flare, often precipitated by a desire to achieve an extremely smooth touch down. A soft touchdown is not essential to a good landing, nor even desired, particularly if the runway is wet and there is risk of Aquaplaning. Trimming the elevator during the flare can contribute to a tail strike. Too much trim can raise the nose and the resulting pitch-up can cause the aircraft to balloon. This is often followed by the pilot instinctively increasing pitch to try to prevent a hard landing. The resulting nose high attitude can lead to a tail strike. A good landing is one made at the right speed and close to the optimum touchdown point at a reasonable rate of descent.
- Crosswinds - On many transport jet aircraft it is possible to deploy roll augmenting spoilers whilst still on the ground during takeoff. If a roll input to counter the increased lift from the into wind wing and decreased lift from the down wind wing is not limited to the maximum advised by the manufacturer (and or licensing authority), the excessive up aileron and roll spoiler(s) deployment will result in an overall decrease in lift generated. To compensate for this a higher than normal angle of attack (body angle) would be required at liftoff which could be sufficient to cause a tail strike. Similarly, on approach and landing a crosswind can increase the risk of tail strike, particularly when conditions are gusty. If the aircraft is placed in an early forward slip attitude to compensate for the wind effects, this cross-control manoeuvre will reduce lift, increase drag, and may increase the rate of descent. If the aircraft subsequently descends into a turbulent surface layer, particularly if the wind is shifting toward the tail, the potential for tail strike becomes high. The combined effects of high runway closure rate, shifting winds, turbulence and the sudden drop in wind velocity commonly found below 100' can make the timing of the flare very difficult.
- Over-Rotation During Go-Around - A go-around initiated during the flare or after a bounce is a common precursor to a tail strike. When the go-around mode is selected, the FD immediately commands a go-around pitch attitude. If the PF abruptly rotates into the command bars, tail strike can occur before a change to the flight path is achieved. Both pitch and thrust are required to execute a go-around, so if the engines are just spooling up when the PF abruptly pulls the nose up, the thrust may not yet be adequate for the manoeuvre. The nose will come up but the tail will also go down and may contact the runway. A contributing factor may be a strong desire of the flight crew to avoid wheel contact with the runway after initiating a late go-around.
Any tail strike can cause substantial damage to the aft fuselage of the aircraft which can be time consuming and expensive to repair. Beyond the cost of the repair itself, further expense will be incurred as a result of schedule disruption and the loss of the aircraft for the duration of the repair interval. A tail strike on landing tends to cause more serious damage than the same event during takeoff. In the worst case, the tail can strike the runway before the landing gear touches down, thus absorbing large amounts of energy for which it is not designed. The aft pressure bulkhead is often damaged as a result. There are several documented cases where improperly repaired tail strike damage has resulted in a catastrophic failure at a later point in time. In the case of Boeing 747 accident, an improperly repaired pressure bulkhead, that had been damaged by a tail strike, lead to the in-flight loss of the vertical stabiliser and subsequent crash of the aircraft seven years later.
Manufacturers have developed their products and their recommended training practices with full consideration given to tail strike avoidance. Aircraft defences include Fly-By-Wire takeoff rotation rate and angle limitations on some types though pilots must understand all fly-by-wire implications; unless the proper procedure for taking control is followed, sidestick equipped aircraft may sum the inputs of the two pilots and on rotation the Sum of 2 inputs can quickly lead to a tail strike. Other defences include tail skids or even small tail wheels to help reduce the amount of damage incurred in the event of a tail strike event. Operational defences include use of higher takeoff flap settings, when appropriate, to increase tail strike protection as well as simulator training profiles inclusive of rotation rate practice, alternate flap takeoff and baulked landing exercises.
Defences against the eight principal causal factors listed above include the following:
- Know your aircraft. A crew that has made a few takeoffs in a given weight range usually knows roughly where the centre of gravity should be and the corresponding trim setting. Test the load sheet numbers against past experience to ensure that the numbers are "reasonable" – i.e. do a ‘reality’ check! Particular care may be required for those conducting Cross Crew Qualification (CCQ) operations; e.g. rotating a heavy A340 Family requires a much firmer back pressure than a light A330 Family.
- Ensure that the computed speeds are correct for the weight and flap setting. Confirm the accuracy of all FMS entries. Do not initiate rotation before VR.
- Use the manufacturer's recommended rotation rate. Where available, use a full flight simulator to practice.
- Do not aggressively rotate the aircraft into the flight director pitch bar. Aim to achieve a speed of V2+10 and the FD commanded pitch at approximately 35' above the runway.
- The single most important defence is to KNOW the tail strike attitude of your aircraft and NEVER rotate beyond it until you are certain you have lifted off (often the air ground sensor relays clicks will give the game away). This guards against both excessively aggressive rotation rates and gross mass calculation error. This may result in a two stage rotation, but that is better than damaging the aircraft.
- Do not continue an unstabilised approach.
- Trim the aircraft in the final approach but not in the flare. Do not excessively hold the aircraft off the runway in an attempt to achieve a smooth touchdown.
- In gusty crosswind situations, actively control the sink rate. Ensure sufficient thrust is available to compensate for slip induced drag and sink rate. If conditions deteriorate below an acceptable level, GO AROUND!
- In the event of a late go-around, minimise the pitch change until the engines have spooled and the aircraft is accelerating. Similarly, if permitted by SOPs, do not retract the flaps to the go-around setting until engines have spooled and the aircraft is accelerating. Momentary touchdown during a go-around is not problematic and the aircraft should not be put at risk by trying to avoid that event.
- In the event of a seriously bounced landing, as opposed to a little skip, a positive Go Around iaw SOPs (sometimes referred to as a TOGA 10, setting power to TOGA, Pitch attitude to 10 Deg) is important to ensure the ground spoilers do not deploy at the peak of the bounce creating a tailstrike and heavy landing.
Accident and Incident Reports
Tail Strike During Takeoff
- A345, Melbourne Australia, 2009 On 20 March 2009 an Airbus A340-500] on a scheduled passenger flight from Melbourne to Dubai suffered a tail strike and did not become airborne until after the end of take off runway 16 at Melbourne at night in normal ground visibility. Only subsequently, when an ECAM (Electronic Centralised Aircraft Monitoring) annunciation of a tail strike was apparent and ATC called, did the crew realise that a tail strike and overrun had occurred and it was decided to return to land to “assess the damage” with a ‘PAN’ subsequently declared. An unusual noise accompanied by a cabin report of smoke in the rear cabin area, just after completion of fuel dumping, led to a precautionary request to ATC for a landing as soon as possible but a full inspection after coming to a stop on the runway founds no signs of fire and a normal taxi in to a gate was then made for disembarkation. None of the 275 occupants were injured but the aircraft rear lower fuselage and ground installations beyond the end of the departure runway struck by it were damaged.
- B763, Manchester UK, 2008 On 13 December 2008, a Boeing 767-300] departing from Manchester for Montego Bay Jamaica was considered to be accelerating at an abnormally slow rate during the take off roll on Runway 23L. The aircraft commander, who was the pilot not flying, consequently delayed the V1 call by about 10 - 15 kts because he thought the aircraft might be heavier than had been calculated. During the rotation the TAILSKID message illuminated momentarily, indicating that the aircraft had suffered a tail strike during the takeoff. The commander applied full power and shortly afterwards the stick shaker activated briefly. The aircraft continued to climb away and accelerate before the flaps were retracted and the after-takeoff check list completed. The appropriate drills in the Quick Reference Handbook (QRH) were subsequently actioned, fuel was dumped and the aircraft returned to Manchester for an overweight landing without further incident.
Tail Strike During Landing
- On 5 February 2010, a McDonnell Douglas MD 81, OY-KHP, on a non scheduled passenger flight from Copenhagen to Grenoble carried out a normal ILS approach to runway 09 in dark night VMC conditions, but the touchdown was made with the aircraft at an excessive pitch angle and higher than normal rate of descent and a tail strike occurred. Serious damage was caused to the rear lower fuselage but none of the 131 occupants were injured and a normal taxi-in and disembarkation followed.
- A321, Manchester UK, 2011 (2) On 23 December 2011, an Airbus A321-200 on a passenger flight from Innsbruck to Manchester sustained a tail strike as the main landing gear made contact with runway 23R soon after the initiation of a go around from a very low height in night VMC following handling difficulties due to the prevailing wind shear. Damage to the aircraft rendered it unfit for further flight until repaired but was relatively minor.
- FOBN "Preventing Tailstrike at Takeoff", Nov 2005.
- FOBN "Preventing Tailstrikes at Landing". Sep 2007.
- IATA Runway Safety Accident Analysis Report 2010-2014 1st Edition, 2015