High Incidence Protection Function (HIPF)
James Albright
Updated: 2020-07-15
Recall that the "angle of incidence" is angle between the chord line of the wing and the longitudinal axis of the aircraft. Further defining the chord line and the longitudinal axis of the aircraft can be a messy process and really doesn't impact us here. What we want to know is why any of this matters to us. The reason for that, is that it matters to the flight control computers.
The High Incidence Protection Function (HIPF) matters to us because it will overrule our actions at times that don't seem to make intuitive sense. It isn't explained at all in the usual places and is only documented incidentally. So what follows are collected notes from various sources, as well as the few places it is actually mentioned in our manuals.
Under contaminated conditions, the HIPF reduces the angle required to get a Pitch Limit Indicator, stick shaker, and other stall prevention measures.
The only real documentation is spread through the AFM:
[AFM, §05-01-01] VSR, REFERENCE STALL SPEED: for the G500, VSR is selected to be slightly higher than the speed at which aerodynamic stall would otherwise occur in 1-g level flight. The HIPF (High Incidence Protection Function) of the G500 limits the angle of attack that can be achieved with full aft stick such that the minimum steady speed is not less than VSR.
[AFM, §05-01-40] Variations of reference stall speeds, VSR, with weight and altitude for speed brakes retracted are shown for all flap positions. The reference stall speeds presented were developed in accordance with 1G stall speed criteria. Because of variation in the stall speeds with the operating mode of the Wing Anti-ice (WAI) system, various charts are presented.
The AFM does not provide reference stall charts for what it calls "Pre-activation Ice Shape" so we are left to guess the stall speed goes up and assume the low speed awareness cues will give us an idea.
[AFM, §05-08-10] Buffet boundary data are presented as a function of speed, weight, and load factor. Buffet Boundary. At low speeds, the buffet boundary is restricted by the maximum achievable angle of attack at full-aft stick due to the High Incidence Protection Function (HIPF). At higher speeds, natural buffet (high speed buffet) or VMO / MMO is limiting.
The AFM gives one chart for 0° flaps that shows buffet boundary increases about 0.02 Mach (approximately 15 knots) at low altitudes with the WAI turned off.
Why does it matter to us?
Note that the HIPF assumes the wing is contaminated whenever the wing anti-ice is off and the flaps are up. That means in this condition the airplane will think it is nearing the stall sooner than is actually true and that can result in the airplane doing things you don't want it to. The Zero or Partial Flaps procedure restricts you to no lower than 200 KCAS "until ready to configure" even with the WAI on. We are taught that this is because of the HIPF but that isn't written down anywhere. The pilot who flew the zero flap tests told me the winglets tend to flutter below 200 knots until the flaps are extended and the flutter can be confused for signs of a stall. One of these days I'll watch the yellow and red low speed awareness bands near 200 KCAS and extend the flaps. Until then, I am using 200 KCAS as my minimum no flaps speed.