FAA Instrument Rating Airplane (IRA)Flight Instruments and SystemsMedium
An autopilot in ALT HOLD mode is coupled to the static pressure system. If the static port becomes partially blocked during cruise flight, which malfunction is most likely?
- AThe autopilot's heading mode will fail completely
- BThe autopilot will disconnect immediately with no warning
- CThe airspeed-based autothrottle will automatically enter overspeed protection
- DThe autopilot will command pitch changes trying to maintain an altitude that is not the airplane's actual altitude
Show answer & explanationAnswer & explanation
Correct answer: D. The autopilot will command pitch changes trying to maintain an altitude that is not the airplane's actual altitude
ALT HOLD mode uses static pressure data to sense altitude deviations. If the static port is blocked, the autopilot receives erroneous altitude information and will adjust pitch to hold what it perceives as the selected altitude, even though the actual altitude is changing.
Why the other options are wrong
- A. Heading mode uses heading/attitude sources, unrelated to the static system.
- B. Most autopilots will not disconnect solely due to a static blockage; they will continue commanding pitch based on bad data.
- C. Autothrottle overspeed protection is unrelated to a static port blockage and not an automatic response here.
Autopilot-Pitot/Static Coupling
Altitude hold and other autopilot pitch modes rely on static pressure data; a blocked static port feeds erroneous altitude information, causing incorrect pitch commands.
- ALT HOLD uses static pressure trend to control pitch
- Blocked static port causes false altitude perception
- Pilots must monitor raw pitot-static instruments even with autopilot engaged
Memory trick: Bad static data in, bad pitch commands out.