Habsheim

[Chris Scott]

Flight on an energy knife-edge

Quote from Dozy Wannabe (my emphasis):
"Theoretically speaking, at 100ft over a flat landscape and with the power set correctly the A320 would be capable of safely maintaining Alpha Max indefinitely until it ran out of fuel."

Theoretically, perhaps... Let's try to examine that in practice.

Quote from gums:
"...the nasal radiators [naval aviators] tend to use throttle for glide path rate of descent to land on a boat. They trim for an AoA/speed and control the flight path with power."

That's rather like most of us are taught to do before our first ever solo-flight on a light, propeller-driven aeroplane - albeit without an AoA gauge. We maintain the correct approach speed by adjusting the pitch-attitude, and adjust rate/angle of descent with throttle/power. (With a tractor-propeller, admittedly, the varying slipstream over the wing itself affects the wing lift slightly.)

Later, in jets, we are taught to adjust the rate/angle of descent with pitch-attitude, while maintaining the correct approach speed with power (thrust) - again without an AoA gauge. The two techniques are, however, essentially the same - achieving a suitable, stable AoA, while varying the flight-path angle (FPA) through the air. Although I've cited the approach case, it's worth pointing out that - in either technique - the FPA does not have to be negative.

How does this relate to Habsheim?

Quote from HN39:
"To achieve alpha-max you have to pull the sidestick to the rear stop and keep it there. To maintain level flight at alpha-max you have to slow down to Valphamax. The only way to 'pull up' in that situation is to increase thrust to accelerate to a speed greater than Valphamax."

So it's clear that the Habsheim scenario - once the stick is fully back, and the AoA is at alpha-max - is critically different from an approach at Vref. In fact, any change of the FPA through the air, as HN39 points out, becomes completely dependent on IAS (strictly speaking, CAS). Any change of FPA requires a change in "normal" acceleration (Nz). In a steady airstream, if that FPA change is to be upwards, the necessary increase in lift can only be achieved by an increase of IAS. That, in turn, can only be achieved by increasing the thrust. On the other hand, descent is not a problem...

Clearly, low flight at alpha-max is hazardous, for at least one reason: wind-shear, which is inevitable over and near trees and buildings, unless the wind at all levels is flat-calm (a rare event). Any loss of headwind or increase of tailwind leads to a loss of IAS. Recovery of IAS requires an increase in GS, i.e., kinetic energy. With no surplus of potential energy to convert, that increase in kinetic can only be supplied by an increase in thrust.

A jet engine will respond fairly quickly when already at a medium-to-high thrust setting (hopefully both engines, symmetrically...), but the pilot's reaction time also has to be taken into account. Arresting the inevitable descent caused by any loss of IAS would take time. There is little or no height to trade for IAS recovery.

To quote again from HN39:
"...with the longitudinal control at the stop it doesn't have the controllability required to maintain 100 ft terrain clearance safely."