How does early rotation increase the likelihood of a tail strike at take off?

Question

This question indicates that an early rotation can cause a tail strike. What are the (pre-)flight dynamics that would cause this to happen vs what happens when rotating at the proper time?

Answer 1

When preparing for a flight, pilots will calculate many things, including their rotation speed. This is the speed in their takeoff roll at which they will start to pitch up, and hopefully the plane lifts off the ground. The rotation speed is dependent on many things such as temperature, altitude, and aircraft weight.

My answer here discusses the lift calculations in relation to rotation. That question covered the importance of speed related to weight. More speed is needed to create the lift to take off with a higher weight.

When the airplane rotates, it increases the angle of attack of the wing, which increases the lift coefficient (if the wing does not stall). The pilots will pitch up at a certain rate, and they do not stop until they reach a climbout attitude higher than what is possible when the main gear is still on the ground (though this may depend on company policy). They count on the plane acheiving enough lift during rotation to leave the ground so that they pass that maximum pitch while in the air.

If pilots underestimate their weight, they will calculate a lower speed than is necessary to provide the lift for takeoff. This means that when they rotate, the plane doesn't have enough airspeed yet to create enough lift to take off. So instead of leaving the ground before fully pitching up, the tail strikes the runway.

It may seem like an easy mistake to make, but thousands of pilots do it every day without a problem.

Answer 2

If you rotate too early (that is, with too little airspeed), the plane will need to reach a higher than expected attitude before it begins rising from the runway.

This means that the plane will be closer to the runway than expected, starting from the attitude where it would normally have lifted off.

Thus, when it reaches an attitude where the tail is lower than the main gear (which is supposed to happen routinely during climbout), it may not yet be far enough above the runway that there is room for enough of the tail below a horizontal plane through the bottom of the wheels.

Answer 3

Aerodynamics (the laws of physics) dictate that when a wing produces 'lift' it necessarily produces 'drag' too. It's not difficult to visualize that a force is produced when a wing moves through the air. Going a step further, the vertically upward component of this is lift, and the horizontal rearward component is drag.

Though these forces are active the moment there is forward motion through the air, the flight controls of large jet aircraft become aerodynamically effective around 60kts, the minimum acceptable inflight effectiveness of controls is deemed to be achieved at a minimum speed starting around 110 to 120 kts and above. (below that, it's still steering and brakes included!)

If the pilots erroneously use speeds based on a lower weight than the actual weight, the ability of the airplane to get the nose up using normally expected control forces is compromised as the airflow over the wings is not producing the required lift (yet), and elevator control effectiveness is also lower, nevertheless whatever lift is being produced does cause the nose to rise. Now the scenario is that, from a frontal viewpoint, far more wing area is being presented to the air because the airplane continues to roll on it's wheels rather than starting to get airborne. This results in a larger portion of the already low (due to low speed) aerodynamic force remaining in the direction of the drag component which naturally impedes acceleration thereby keeping it below the correct rotation speed for even longer.

The early rotation can be visualized as a wing mushing through the air and churning it up rather than flying through it in a streamlined manner with a relatively smooth airflow all around.

Now the angle of attack of the wing (~ pitch) has has to be increased still further to produce enough lift to counter the twin effects of insufficient lift due to lower speed, as well as higher drag component. The airplane runs the risk of getting airborne in a high angle of attack high drag configuration seeking even higher pitch - so all around there's a tendency for high pitch and the possibility of a tail strike.

In the earliest days of jetliners, many went off the end of the runway, getting caught out by the compounding of these 2 effects and engine thrust limitations. Those accidents signaled the need for clearcut requirements by the regulations, of take-off calculations based on performance requirements.

The performance calculations do have some inbuilt margins, so the chance of a tail strike if they've calculated the speeds for say 202tons rather than 220tons is less than if they've used speeds for 230tons instead of 320tons.

Be clear that there is no subjective element in take-off procedure and technique. To achieve the legally required take off performance the the thrust setting and speeds are calculated from approved published data and this is only valid provided the airplane is operated as described in the book.

Gross error such as wrong weights used can only be guarded against by doings checks and cross-checks at every stage.