Tail strikes: What are they, why are they caused?

The audit was commissioned after IndiGo witnessed four tail strike incidents on its A321 aircraft within a span of six months this year, with the latest being during a landing at the Ahmedabad airport on June 15.

In the release, the DGCA said that during the audit, it reviewed the airline’s documentation and procedure on operations, training, engineering and FDM (flight data monitoring) programme.

What is a tail strike?

A tail strike refers to an incident where the tail of an aircraft hits the ground or strikes any other stationary object. While tail strikes can occur during takeoff, a majority happen during the landing of an aircraft. According to Airbus statistical data, over 65 per cent of tail strikes happen during landings.

Tail strikes can cause significant damage to the aircraft, with major repairs needed to restore the plane’s structural integrity. Even in cases where the damage is not major or immediately obvious, thorough inspections are carried out before the aircraft is declared fit to fly again.

What causes tail strikes?

While modern aircraft are fitted with a whole gamut of systems to aid pilots in flying aircraft and reduce the probability of human error, most tail strikes can be attributed to mistakes made by pilots.

Simply put, tail strikes occur when the pitch attitude of the aircraft (more on that, in a moment) – while taking off or landing – is steep enough for the tail of the craft to hit the ground. Aircraft, depending on their size, have different “tail strike margins” – the longer the aircraft, more prone it is to a tail strike as the rear of the plane juts out further behind the rear undercarriage.

Before going into greater detail on the specific reasons, a look at how an aircraft moves.

Yaw, roll and pitch

An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; roll, rotation about an axis running from nose to tail; and pitch, nose up or down about an axis running from wing to wing. Look at the animation below for more clarity.

(Source: Wikimedia Commons)

These are collectively known as an aircraft’s attitude.

Tail strikes are most impacted by the aircraft’s pitching motion. A positive pitching motion raises the nose of the aircraft and lowers the tail. Tail strikes are caused by such a motion being executed improperly during take off and landing.

Tail strikes during take offs

There are a few different reasons why tail strikes occur during take off.

• Incorrect takeoff speeds: Each aircraft is designed to takeoff at a certain speed, known as Vr or rotation speed, which varies, depending on the weight being carried. Below this speed, its wings cannot produce sufficient lift to become airborne. If a take off is attempted while the plane is still slower than this minimum speed, the thrust from the engine will push the nose upwards, but without sufficient lift from the wings, the tail will simply hit and drag on the ground. Most often, this happens due to a miscalculation of the Vr made due to incorrect input of the aircraft’s weight by the pilot.
• Poor rotation technique: At Vr, pilots pull back at their stick, inducing a positive pitching motion, to achieve liftoff. However, the stick cannot simply be yanked back – the pilot has to pull it in a measured way so as to take off at the right angle. An excessive pitch rate can lead to a tail strike – if the nose is too high while the plane is not sufficiently off the ground, the tail will strike the ground.
• Incorrect centre of gravity, or mistrimmed stabiliser: An aircraft’s centre of gravity location determines its handling. If the nose side is heavier, pitch control becomes less sensitive. If the tail side is heavier, it becomes more sensitive – thus making it susceptible to mishandling by the pilot due to overcontrol. During pre-flight, the pilots set the stabiliser of the aircraft to account for the aircraft’s centre of gravity. An error here can also cause a tail strike.

Tail strikes during landing

Tail strikes during landing are more common, and generally also cause more damage. This is because, during landing, if the tail strikes the ground before the landing gear, it absorbs a majority of the energy of the impact of the aircraft with the ground. Such tail strikes are caused by unstable approaches.

A stable approach is one where the aircraft approaches the runway at the right speed and with the correct glide ratio – the distance of forward travel divided by the altitude lost in that distance. Unstable approaches can cause tail strikes due to a few different reasons.

• Too low an approach speed: During landing an aircraft, if the approach speed is too low, the aircraft will sink too quickly. This is because it is the speed of the aircraft which keeps it in the air by producing lift through its wings. The only way to reduce this sinking is to pull on the controls harder, which increases the pitch attitude of the aircraft – consequently reducing the tail clearance and making a tail strike more likely.
• Too high or too low of a landing flare: Landing flare refers to the manoeuvre undertaken by pilots to land the aircraft – the nose of the plane is raised and the rate of descent is slowed to achieve an optimum landing. Initiated too early, the aircraft will lose speed leading to complications described in the previous point. Initiated too late, pilots will have to increase the pitch attitude to slow down quicker – again creating a situation where the tail clearance is reduced, thus potentially causing a tail strike.
• Incorrect handling during crosswinds: Crosswinds strike aircraft perpendicular to the line or direction of travel, on either side – basically wind blowing across the runway rather than along it. Landing during crosswinds is much harder as the pilot has to maintain a stable approach to the runway while being pushed away from it due to the winds. To do this, roll controls are used alongside the standard pitch controls used during takeoff and landings. However, roll spoilers used during such landings also impact the lift generated by the wings of an aircraft – leading to a greater sink rate which, again, increases the chances of a tail strike.

How to prevent tail strikes?

Modern aircraft also have sophisticated software which detect the tail clearance and adjust control accordingly. For instance, Primary Flight Computers (PFCs) of the Boeing 787s monitor the tail clearance at all times – below a certain threshold, the aircraft automatically adjusts the controls without direct input from the pilot.

However, the most critical component to avoid tail strikes remain pilots themselves. Proper training of pilots and rigorous implementation of correct flight procedures are the single most important factor to avoid tailstrikes. All the common reasons for tail strikes described above can be mitigated by pilots, whether it be in their preflight preparations or inflight handling of the aircraft.