737 Lav Flush/Cabin Diff
 

I've been whiling away another boring lockdown day by watching one of the excellent Pilots Eye videos, this one following a crew in an Air Berlin (ah the memories) B737-800 from Munich to Hurghada. They were discussing pax moving about the cabin and the resulting effect on trim as they get up and down to use the lav, the joke being 'the worse the food, the more the trim'. Who says Germans don't have a sense of humour? :)

They also pointed to the cabin diff gauge on the overhead and showed the needle climbing up a little before returning to normal, saying that when the lav was flushed that cabin air was vented overboard. I didn't quite understand this, but this was with English subtitles of German dialogue so maybe something was lost in translation. I know that the 737 uses a vacuum lav, but how is that connected with the outflow valve? Is the air that gets sucked into the vacuum lav vented overboard? I've never heard of this one before, so was curious.

Thanks in advance. RU

without consulting the books, the basic principle is to :
1. use diff press above 16000ft to 'push', or 'blow', the poo from the bowl down to the waste tank, then separate the poo (waste) at the tank, and discharge the stinky air overboard via an orifice we called 'the poo chute', typically on the left hand side at the back - the orifice with brown streaks coming from it (insert spewing face here)
2. use the 'blower' to create a vacuum below 16000ft to 'suck' the waste down to the tank, then separate the poo (waste) at the tank, and discharge the stinky air overboard via an orifice we called 'the poo chute', typically on the left hand side at the back - the orifice with brown streaks coming from it (insert spewing face here)

The outflow valve has nothing directly to do with water/waste management.

The pressure sensor for the ovhd pnl cab diff press is, from memory and dont quote me, on the back of the gauge itself. If not, its in the equip centre somewhere.

It is possible that the cab diff press sensor can sense the subtle cabin pressure change due to toilet flush (an induced pressure leak). I would have thought, that the flush would cause the needle to dip to a lower pressure momentarily, however, there might be a phenomena/characteristic that im forgetting that causes a sudden rise in pressure when the flush valve changes state from closed to open.

hope this helps... i dont like to make over complicated explanations

Makes sense, thanks for the comprehensive reply!

Regarding pax moving about affecting the trim; my first commercial flying job was on the Shorts 360; the "Shed". No autopilot, hand flown all the time. One could feel the change in pitch as our cabin crew walked from the galley at the back to the cockpit - requiring slight extra back pressure on the yoke to stay level. If I was flying I could say to the Captain, "Our coffee is coming". "Eh?" they would say, before the cockpit door slid open and our hostie would be there; " hello boys, here's your coffee".

Nice. I never got to fly on a Shed, although there were plenty passing through my local airport - Liverpool. The Shed did have an autopilot though, right?

Not the ones we flew, hence we could feel the coffee approaching the cockpit :)

No flight directors either, it was all hand flown, properly trimmed and relying on your instrument scan. Great for consolidation of basic flying skills.

IIRC, later examples had F/D, but still no A/P.

Gotcha. I worked with a lass years ago who was cabin crew on Sheds and she said the big boxy fuselage made it a nice spacious aircraft to work in. She went onto the EMB 145 sometime after, which it appears was quite the opposite!

Perhaps when the pilot on the video said the gage briefly rises with a flush they were talking about the cabin altitude change rate or cabin pressure altitude needle rather than the differential pressure needle?