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fsgeek

Curious chain of events

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Hi everyone,

 

Something interesting happened to me when flying a Thomson 737-800 to Skiathos. The outside air temperature before take-off was 30C which didn't make me think twice. A couple of seconds after pressing TO/GA I got a master caution. I couldn't see the message from my side so I assumed it was the door lock glitch. However, the light remained on. It was an "Air cond" caution and the bleed trip off light was illuminated on the overhead panel, along with a cabin zone temp warning for the aft cabin. The temperature for the aft cabin was at 31C even though the temperature control was set to auto. I decided to set CMD B so that I could take a closer look without having to worry about crashing the plane, only the A/P immediately tripped onto CWS P mode and VNAV also tripped with LNAV remaining on. The aircraft continued to accelerate towards 300 KIAS even though the A/T was engaged and 154 KIAS was set on the IAS/Mach window of the MCP.

 

My question is: What caused the air con caution? Also, could the issue with the A/P be related? I had the FMS properly programmed with SID information entered along with everything else necessary to use the A/P fully.

 

Thanks,

 

Louis Vallance

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Were your recirculation fans turned on?

 

When you engaged CMD B, did you verify LNAV and VNAV were engaged?

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My question is: What caused the air con caution?

 

Cabin was too hot.  Try the reset button on the Pack Panel to see if it kicks the light out.  If you didn't spin the APU on the turn (or with enough time prior to push) the cabin probably didn't have a chance to cool.

 

 

 


Also, could the issue with the A/P be related?

 

I'd argue no.  The aircon doesn't interact with the AP at all.  Sounds like a separate issue, and one likely related to you trying to engage the AP outside of tolerances.  If you're using "Realistic AP Engagement" (PMDG Options), take a look at the Intro Manual to get a brief overview of what all that entails.

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Thanks Kyle, that clears it!

 

 

 


When you engaged CMD B, did you verify LNAV and VNAV were engaged?

 

Yep, both VNAV and LNAV were armed prior to CMD B being engaged.

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CWS mode.

 

means the yoke (joystick?) was moved after the autopilot was engaged usually. 

 

This means now that vertical navigation is disenaged and the aircraft will maintain the current pitch. If it's soon after takeoff you likely have N1 mode on the Autothrottle. This will main climb thrust, expecting VNAV, FLCH or your yoke to adjust pitch for speed.

 

CWS means that the autopilot will maintain pitch (so it will trim down as you accelerate automatically) unless you pull/push the yoke around.

 

You were basically flying Climb Thrust with manual(ish) control of pitch. In that configuration it is possible to accelerate to VNE and beyond without any protections. It's also possible to decelerate to stall and beyond if you pull the thrust back to idle in CWS.

 

Back in the old days of 737-200's, CWS was one of the only autopilot climb modes, It's a legacy mode that's kinda been left there. (The Sperry 77 autopilot of old didn't have the technical ability to maintain a specific ft/min climb rate, just a pitch, which was adjusted by the yoke and maintained through automatic application of trim. When the 737-200 with a Sperry 77 autopilot got to the cruise altitude, the pilots would push the yoke forward to reduce climb rate, then at the required altitude, use their fingers to change from CWS to ALT-HOLD, which would hold the altitude they were at when the button was pressed. ie 31,008ft or 30,997ft etc).

 

You should be able to re-engage LNAV/VNAV from CWS mode unless you have broken the Autopilot servos (with a really fast/big push of the yoke with the autopilot on for instance.)

 

Disengage/reingage autopilot if required to do this.

 

btw, when you went into CWS mode you should have got an audio warning similar to the autopilot disconnect that would have only made 2 warbles (instead of never-ending-till-you-double-tap warbles).

 

CWS = Control Wheel Steering.

In this mode you use the Yoke to make the autopilot move to a new attitude, and once the yoke is centred, the autopilot will keep that attitude till you change it/change modes/the plane stalls or highspeed buffet drops the autopilot out. The autopilot will even pass through the set MCP altitude without flinching in CWS mode. The only indication will be the Altitude alert horn as you approach, and then pass, the MCP selected altitude.

(note: terms "Attitude" and "Altitude" are not the same thing.)

 

There may be other times the CWS mode engages, usually double-pressing VNAV. (Selecting VNAV, then a second press which selects "Autopilot still on but no vertical modes selected")

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The autopilot will even pass through the set MCP altitude without flinching in CWS mode. The only indication will be the Altitude alert horn as you approach, and then pass, the MCP selected altitude.

(note: terms "Attitude" and "Altitude" are not the same thing.)
I know that's been said, but I've seen it lock on alts. I don't think it will do that. Maybe the -200 would but, I don't think that's true anymore. I'll have to dig some before I'm 100% sure, but I'm 99% sure I've seen otherwise.

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I know that's been said, but I've seen it lock on alts. I don't think it will do that. Maybe the -200 would but, I don't think that's true anymore. I'll have to dig some before I'm 100% sure, but I'm 99% sure I've seen otherwise.

That was with a few local brews last night, so I wasn't very clear.

 

However, it seems that CWS can honor MCP alts depending on how you enter CWS. I think if it's by going directly to CWS via the CWS button, it might bust alts, but if you enter by force on the yoke or via deselecting another pitch mode it will ALT ACQ. It might be that there shouldn't be force on the yoke, as well. (Not sure how the NGX would handle that. . . )

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"Bleed Air Trip Off" should occur, if the bleed air entering the left and right ducts for "Wing-Bodies" and "Packs" is warmer than 230°C.
The bleed air is a mixture of engine compressor air (hot) and fan air (cold). On very hot days this mix might fail to be below 230°C. Than the bleed air trip into wing-bodies and packs is blocked.
While on ground on hot days (over 24°C) it is a good idea to pre-cool the cabin via the APU+Packs (at high) on 21°C and keep it cooled until before T/O.
On hot-day-T/Os I turn off the bleeds for thrust preservation (instead of the packs on normal or cold days > bleed air could be needed for wing-anti-ice!) and at the same time for bleed-air-trip-off avoidance. < sorry, but could make no sense: it might always light on when bleed air "knocking at the duct door" is warmer than 230°C. I have to check that again...<

^^^^ just checked on PMDG-NGX-FCOMv2, 2.20.1, p133:

(Quote:)
Bleed trip sensors illuminate the respective BLEED TRIP OFF light when engine bleed air temperature or pressure exceeds a predetermined limit. The respective engine bleed air valve closes automatically.

(quote end)
Ram doors will remain open on ground until full flap retraction in the air and have the task to supply the packs (pressurizers & air conditioners) with air flow for pressurization support and heat exchanging. Heat exchanging for cooling works better with cooler ram door air. So on very hot days cool pack-air outflow is demanding. Packs "on high" reduces T/O performance due to increased drive generator load.
So it might be a good idea to keep the APU on for the "zone temp" lights to stay off.

AND NOW TO SOMETHING COMPLETELY DIFFERENT:
I checked the A/P behaviour in the NGX -800 that was described by Louis.
My only explanation is that the A/P in A/T+VNAV+LNAV mode was overridden by yoke pitch input. So the A/P downgraded pitch by setting CWS P and cancelling VNAV.
This way pitch is manually up to the pilot and VNAV remains on N1 for climb until ALT ACQ is reached. This leads to overspeed, if pitch remains too low... (speed on pitch)

Greetings,
Claus

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This way pitch is manually up to the pilot and VNAV remains on N1 for climb until ALT ACQ is reached. This leads to overspeed, if pitch remains too low... (speed on pitch)

One more correction on that:

This way pitch is manually up to the pilot and A/T (not VNAV) remains on N1 for climb until ALT ACQ is reached. This leads to overspeed, if pitch remains too low... (speed on pitch)

 

Sorry for being inaccurate!

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