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Noble.

Slightly bizarre autopilot behaviour?

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Firstly, I've tried searching the FCOM vol. 2's autopilot section to find an answer to this, but haven't been able to find anything really.

 

I was in the descent (approach technically - BUBIN 6A into OMDB) and was perfectly on the descent path and LNAV. I reached GIRMI where there's a 230 knot speed restriction, so we slowed from ~260 knots down to 230 knots all perfectly planned for in the FMC. However, when we got down to 230 knots, I decided I wanted to go back to 250 knots as we still had quite a few track miles to go. So I intervened by clicking on the IAS/Mach knob and dialing it to 250 knots. This is the first interesting thing that happened, I was expecting the autothrottle to switch from HOLD to SPD to increase the throttles - however this didn't happen, instead the aircraft essentially went into FLCH mode and dropped 1,000 ft below the descent profile to gain the 20 knots - all while in VNAV (I know this may not actually be an issue, although it seems somewhat counter-intuitive to do this, it also isn't what happens on the NGX when do speed intervene but different aircraft I know). After this, it didn't correct itself, it continued to drop further and further from the descent path to maintain speed (as if it were in FLCH mode) all while in VNAV. Finally, when we were ~2,000ft below the descent profile, I hit the altitude HOLD button and waited for the descent profile to catch up. When it finally did, I switched the aircraft back into VNAV - where it commanded to slow down from 250 knots to 185 knots, however, the aircraft decided to throttle all the way up to full CLB power (~93% N1) and try to climb until I intervened a few seconds after (although by this point we were getting ready to contact the Houston space centre after experiencing 93% N1 150 tonnes below MTOW)

 

Just some additional information, I have my joystick set up so that it is never allowed to takeover from the autothrottle (due to there being noise in the joystick) so this isn't an issue where the autothrottle mode changed and commanded my joystick throttles.

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It probably switched from VNAV PTH mode to VNAV SPD mode on the FMA?

 

I don't remember unfortunately. 

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The behavior is consistent with SPD mode. Take a look at the manuals.

 

Where abouts in the manual? I read through the FCOM volume 2's autopilot chapter and read nothing that would support either of the behaviours I saw (especially not the latter).

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Firstly, I've tried searching the FCOM vol. 2's autopilot section to find an answer to this, but haven't been able to find anything really.

 

I was in the descent (approach technically - BUBIN 6A into OMDB) and was perfectly on the descent path and LNAV. I reached GIRMI where there's a 230 knot speed restriction, so we slowed from ~260 knots down to 230 knots all perfectly planned for in the FMC. However, when we got down to 230 knots, I decided I wanted to go back to 250 knots as we still had quite a few track miles to go. So I intervened by clicking on the IAS/Mach knob and dialing it to 250 knots. This is the first interesting thing that happened, I was expecting the autothrottle to switch from HOLD to SPD to increase the throttles - however this didn't happen, instead the aircraft essentially went into FLCH mode and dropped 1,000 ft below the descent profile to gain the 20 knots - all while in VNAV (I know this may not actually be an issue, although it seems somewhat counter-intuitive to do this, it also isn't what happens on the NGX when do speed intervene but different aircraft I know). After this, it didn't correct itself, it continued to drop further and further from the descent path to maintain speed (as if it were in FLCH mode) all while in VNAV. Finally, when we were ~2,000ft below the descent profile, I hit the altitude HOLD button and waited for the descent profile to catch up. When it finally did, I switched the aircraft back into VNAV - where it commanded to slow down from 250 knots to 185 knots, however, the aircraft decided to throttle all the way up to full CLB power (~93% N1) and try to climb until I intervened a few seconds after (although by this point we were getting ready to contact the Houston space centre after experiencing 93% N1 150 tonnes below MTOW)

 

Just some additional information, I have my joystick set up so that it is never allowed to takeover from the autothrottle (due to there being noise in the joystick) so this isn't an issue where the autothrottle mode changed and commanded my joystick throttles.

 

Hate to say it, but this is all user error here.

 

When you open the MCP speed window outside of approach mode, VNAV reverts to VNAV SPD. VNAV SPD is a pitch-for-speed mode and for all intents and purposes identical to FLCH. You give the airplane a speed and it pitches down to maintain it while holding thrust at idle. Unless you see VNAV PTH annunciated, you are not actually following any sort of defined path at all. The NGX works exactly this way too by the way, so I'm not sure what you're getting at there.

 

Approach mode gets defined by a bunch of different things, but the big one is having the flaps out. Once you're in approach mode VNAV will stay in PTH mode with the speed window opened and will follow the path regardless of airspeed.

Where abouts in the manual? I read through the FCOM volume 2's autopilot chapter and read nothing that would support either of the behaviours I saw (especially not the latter).

 

As far as I know every Boeing autopilot behaves this way.

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