Boeing 737-200 Sperry SP-77 Alt Hold

[Moor337]

Could anyone tell me what happens when the captains ALT HOLD switch is on but the F/O's ALT HOLD switch is off on the control panels of the Boeing 737-200 Sperry SP-77 Auto-flight System?

[vdbroeck.net]

This was the original MCP as fitted to the -100/-200's with the Sperry SP-77 autopilot. They comprised three panels, the autopilot (centre) and one for each flight director (sides). All three were independent so any mode to be used with the autopilot (eg ALT HOLD or VOR/LOC) had to be selected three times. HDG SEL and VOR/LOC could be coupled to the autopilot but would only be driven by the heading bug set on the Captains compass and the course set on the Captains HSI.

[Moor337]

Hi, vdborek.net

thanks for your quick reply. Just to be clear the switch i am on about only has an ON/OFF function i believe you are on about the display selector with the VOR/LOC etc. i will put a picture below to try and make it clearer.


so will the ALT hold function in this config or not.

thanks.

[Tundra Tire]

That switch is only for the flight director and has zero effect on the actual autopilot system.

For example, when either pilot selects a roll mode on their flight director (hdg or vor/loc), then lateral info is provided to the fd, but no vertical info. The pilot would then have to manually adjust the pitch cmd dial just to the right of the alt hold switch which would artificially set your fd to whatever pitch attitude you desired. By selecting alt hold on the fd panel, the fd would now get vertical info. For obvious reasons, it only provides useful info when in alt hold mode on the autopilot (level flight) or in auto app mode (ILS approach).

There is also no effect to the system by one fd being selected on and the other off, other than the two fd's may be showing different pitch information.

[A37575]

The pilot would then have to manually adjust the pitch cmd dial just to the right of the alt hold switch which would artificially set your fd to whatever pitch attitude you desired.

Which is why using the FD on the 737-200 is a waste of effort when you have to fiddle around with the pitch command dial so it can tell what you already know

[Tundra Tire]

In the climb, or descent, absolutely.

It's only really useful when the autopilot is in alt hold or auto app mode.

[Moor337]

That wraps it up, thank you all for your replies as this helps me with an assignment.

Cheers.