Yesterday I was flying a Lufthansa B747-400 from Cologne to Amsterdam. On final approach to Schiphol, as I set my airspeed to 200 knots, I noticed that my ASI was stuck at 236 knots. Even as I set it further lower to 180, the ASI wasn’t responding. So I disengaged it and manually used the throttles to land. The landing was fine. But is this a glitch ? Certainly it isn’t like a real world pitot tube issue ? What freaked me out was that only yesterday I was re-watching the mayday episode of Aeroperu 603. Same thing there (Also for Birgenair 301, although they both were 757s). Malfunctioning ASI !!
You were piloting a heavy aircraft and your rate of descent probably caused you to maintain or not be able to lose airspeed. You can use your spoilers to shed some of that airspeed. Autopilot will not arm them for you.
Mine has done it a few times. I’ve also had instances in the F14 where conditions seems to mimic a pitot tube error, with the airspeed then responding like the altimeter
The AP speed being set does not force your plane to slow to that speed. If you are descending too fast it will pick up speed even though your throttle is at 0.
But I wasn’t descending too fast. I was descending at only -300 ft/min. Then why did it malfunction ?
It depends on your plane, weight, and other factors. Some planes are just harder to slow down. AP does not apply spoilers to slow you down, it simply puts the throttle down until you reach that number.
The only mention I saw of the throttle was when you mentioned switching to manual.
The one indicator that the AP was attempting to maintain the incorrect speed would be if the throttle was not idling. If the throttle was at idle and you weren’t losing speed, then it was due to the other factors already mentioned.
Unless you know or can show that the throttle was at a significant deviation from 0% (i.e. actively maintaining the wrong speed), there’s no reason to suspect that there is anything significantly wrong or easily reproducible.