By Captain Shem Malmquist
One area of flight performance that is misunderstood by some pilots is the affect of pitch changes just prior to landing. Specifically, some pilots will aggressively lower the nose just prior to touchdown. While this technique will result in getting the aircraft on the ground in a hurry it also has some negative consequences. Before discussing these issues, let us first dispel some of the misconceptions surrounding this issue.
The first misconception is that lowering the nose of the airplane will “raise the landing gear” thus resulting in a smoother landing. While it is true that the geometry of the airplane does result in the main gear effectively moving up relative to the center of rotation, the effect is not significant. Obviously the longer the airplane the more pronounced this effect would be, so we can use a large wideoby as an example as a “worst case”. The main gear on the large widebody is about 8 feet aft of the center of the CG range. Using basic trigonometry we find that even an 8º pitch change only results in a net 1 foot movement of the main landing gear.
The second aspect that is sometimes raised is that forward pressure on the control column reduces the total load on the wing as less tail down force is required, resulting in more lift to slow the rate of descent. While this does have some merit over very small pitch reductions (less than 1º or so of pitch change), it requires precise timing and the possible negative affects of larger pitch reductions can be catastrophic. To understand the reasons for this some background understanding is required.
For practical purposes, in stabilized flight all of the forces acting on the airplane are equal. As long as the aircraft is not accelerating its rate of climb or descent and the wings are level (assuming no turbulence, etc.) the g meter in the airplane will be reading 1.0 g. This is true whether the airplane is maintaining an altitude or in a constant rate climb or descent. If the rate of climb or descent is changing then the g meter will be registering a number either above or below 1.0. If it is in a descent, a reduction in the rate of descent will occur with an increase in g to some value over 1.0, and if the rate of descent in increased the g meter will show some value less than 1.0 as the aircraft temporarily accelerates to its new state of equilibrium.
Aircraft landing gear is designed to absorb load in two parts, which we can call “Phase 1” and “Phase 2”. Phase 1 would be the force due to the vertical motion of the airplane. If you do a “carrier landing” the landing gear is effectively stopping your vertical speed. The wings are supporting all of the weight of the airplane until the nose is lowered, spoilers deploy and the airspeed is reduced. Instead of this technique, most pilots try to smooth out the landing by increasing the lift a bit just prior to landing, commonly referred to as the “flare”. If you were to be watching the g meter during the flare you would see it register something above 1.0 as the wings accelerated the airplane upwards (note that an acceleration upwards in this case means actually a reduction in the downward vertical speed and not actually motion away from the ground). Any remaining vertical speed will be absorbed by the landing gear as part of Phase 1.
Phase 2 is the weight on the landing gear that is present when the airplane is sitting on the ramp. The basic force of gravity we all feel. During landing this portion is not ordinarily a factor until after the aircraft has been derotated and the lift from the wings removed. The design criteria for the landing gear assumes that Phase 1 and Phase 2 do not happen concurrently – that Phase 2 only occurs after any vertical speed is arrested. If the nose is lowered prior to touchdown it can result in the wings not supporting the weight of the airplane prior to touchdown, thus adding some portion of the Phase 2 force to the initial touchdown.
Another way to describe this issue is by referencing the g meter. Recall that the g meter uses 1.0 as the “baseline”.
The total amount of force absorbed by the landing gear can roughly be described as the difference or ∆ between what the g meter is reading just prior to touchdown and what it reads as it touches down. If you are reading 1.1 just prior to touchdown and you touchdown with a net zero rate of descent the g meter not only will not increase, but will actually move down towards 1.0 as the acceleration stops. This feels very smooth. If you read 1.1 and touch down with some remaining vertical speed, the g meter may “spike” upwards a bit to stop the motion. In a firm landing the g meter might go (for example) from a reading of 1.2 to 1.7, and the net of + 0.5 would be the force you and the landing gear “feels”.
If instead you pitch the aircraft over just prior to touchdown the force the landing gear “feels” is measured from the ∆ (change) of what ever point you started with to what you ended with . For example, say the “pitch over” results in the g meter registering 0.8 g just prior to touchdown and 1.1 at touchdown. For practical purposes the landing gear will now absorb 1.4 – 0.7 = 0.7 g. Although 1.4 g in this case is a lower value than the 1.7 g in the above case, the net g of 0.7 g is higher than the net of 0.5, so the latter landing would feel harder and would be harder on the landing gear.