Envelope protection methods
Avionics mfrs build autopilots to address inadvertent unusual attitudes and overloads.
Citation Ten has full envelope protection provided by Garmin's latest G5000 avionics system.
The system turns the aircraft 90°, engages the autothrottles to reduce thrust to idle and descends the aircraft at 10 kts less than maximum down to 15,000 ft where it levels out, still on autopilot, waiting for the crew to regain control.
Garmin's system has an emergency descent mode that has the same functionality as the Honeywell system, but Garmin goes a couple of steps further. Whether on autopilot or not, the system can determine if the crew is incapacitated.
On autopilot, if there are no frequency changes or other interactions for a certain period of time, an annunciator alerts the pilot and he/she can then effectively reset the timer. Off autopilot, if ESP is active for a predetermined percentage of the time, the autopilot engages and brings the aircraft back to straight-and-level.
If the system detects incapacitation or sudden depressurization at higher altitudes, an emergency descent is initiated.
Asked if this emergency descent mode is integrated with the TAWS to avoid high ground, Garmin's Mead only replies, "Not yet."
The end result is that, if the pilot loses control, either through upset, disorientation or incapacitation, Gamin's newest system will restore the aircraft to "blue side up." In fact, the system has a function (only implemented on some Cirrus aircraft) that, at the push of the blue LVL button, restores the aircraft immediately to straight-and-level flight.
Sophisticated envelope protection systems, either stand-alone or part of a FBW package, can provide a real benefit in terms of increased safety and operational efficiency in all aircraft and all operational environments—but it's vital that pilots understand the different modes and how they function in different flight regimes.
It is also incumbent on manufacturers to make these systems understandable and consistent so that their actions can be anticipated and recognized in flight. Remember the old joke about the phrase most often heard in the cockpit. It used to be, "Where are we now?" but it has become, "What's it doing now?"
Mike Venables is an aviation consultant and freelance writer. The principal at TriLink Technologies Group, Venables has been involved in the aerospace industry for more than 40 years, including aero engine, airframe, avionics and simulator manufacturers.