Foxcotte

I'm hoping there is someone out there who can settle my curiosity...

I flew from Nairobi to Jo'burg on Monday this week on an SAA 737-800. The aircraft was only 1/3 full, and everything was absolutely normal until we were top of climb. Then I got a few 'ear pops' which was the first clue something odd was up. Then there was a substantial power reduction, power back up, power reduction and the aircraft was fairly steeply descended. I'm not a big jet pilot but from experience flying a corporate jet it all seemed rather odd. Eventually we leveled out somewhat lower than original, but there was a serious out of sync vibration in the cabin. This vibration continued on and off for the next hour at least, with various power changes as the crew tried to sort something out. The engines stabilised for 15 minutes at a time, then went out of sync again etc. After sometime the flight deck explained that they had had a problem with pressurisation at 38,000' so had come back down to 28,000' and were continuing onto Jo'burg as scheduled. The rest of the flight was normal, except we seemed to have several large changes of heading which I don't recall from past flights on the same route.

Lastly, there was a few minutes when the cockpit door was ajar and we could see a bank of annunciators in the centre panel that seemed to have at least 3 amber and 1 red lights showing on the top row?. Again I know nothing about the 737 but small jets normally operate with a black/clear annunciator panel.

As a postscript to the above, by coincidence a friend was travelling the reverse route the very next day on SAA, and shortly out of Jo'burg the crew on the 737-800 announced they had a pressurisation problem and turned back to Jo'burg at FL080' all the way back in. He waited a while and got put on another aircraft to Nairobi a few hours later.

So, can anyone out there shed any light on what was happening, what went wrong or why?? I'm just curious more than anything else. I'm also guessing that there are very different SOP's for pressurisation issues when heading inbound to home-base than when outbound. But I can't help wonder if it was the same aircraft and the problem hadn't been rectified satisfactorily after the inbound flight....

I.R.PIRATE

I think your GnT was a tad strong Mr.Fox.

SAA does not have incidents.

I repeat, does NOT have incidents.

Now keep a close watch for those black 500SL Mercs with 100 years of selfless struggle written on the side. They are coming for you.

Next Leg Undefined

Hi Foxcotte, unless you're lucky enough to have one of the crew of that flight (or someone in the know) view your post and respond you will have to wait for the monthly safety report summary which is sent out to all the crew. That will be at the end of Dec or, maybe due to the holiday season, the end of Jan. If you don't get a technical response to your post it is probably because no-one knows, yet. I am sure you will have non-technical responses...

Temp Spike

Not a lot of usable data there, and, I‘m not 737 ‘spert, but sounds like a stuck pressurization outflow valve. They were probably to cheap to change it, opting to monkey around with it and see what happens. Could also have been a leaking door seal,

Red light…hmmm…not a friendly sign that. Cargo door not fully closed and locked? Red on annunciator could be several things as indicated in the link.

Amber - auto pressurization system fail.

Anyway here is a link to the annunciator panel cue cards for the 737-800.

Boeing 737-800 Annunciator Lights flashcards | Quizlet

Should be noted that repairing aircraft is done on the ground by engineers not by pilots in flight. Crew reaction was stupid IMO.

Shrike200

Red lights in 737's generally means something caught fire. There's a loud bell to wake the crew up from their slumber too. Or gear config/disagree lights I suppose, if the thrust levers were at idle when you saw the lights. The SOP's don't change just because you're heading home, just the decisions on how best to proceed. Sounds like the pressurisation controller just misbehaved a little and didn't do it's job. You'll end up doing something like Auto fail -> standby mode (which is just the other side auto controller in the -800) -> manual mode and fiddle with the outflow valve -> rapid descent if you couldn't control it very soon. Something like that. So it doesn't sound all that major to me.

Trossie

"Red lights in 737's generally means something caught fire."

-- or, thrust levers at idle in a descent with gear not locked down?

"...there was a few minutes when the cockpit door was ajar..."

-- interesting from the security side of things?!

Shrike200

My intention was to say that it either probably wasn't a red light, or they were descending at idle thrust and the gear warning lights were on (totally normal). Sorry if it wasn't clear.

And yes, occasionally the door is opened to check if the bus drivers haven't died in the meantime if the head honcho in the back feels like looking at them instead of phoning them. I doubt he seriously meant the door was left wide open for minutes. Normal stuff.

Occasionally there are hassles with the pressurisation that don't get diagnosed/fixed straight away if it's some weird fault that is difficult to trace. Then the engineers start swapping bits out to try and localise the problem, but occasionally they guess wrong the first couple of times, and/or spares aren't forthcoming. Sounds like one of those times.

Romeo E.T.

air/ground sensor failure, can cause lots of strange problems, take-off config warnings (red), pressurization problems etc....just thinking aloud !

Foxcotte

Thanks for the thinking so far...

But it was definitely a red annunciator of some description. And definitely when we were in sustained level cruise somewhere over Malawi.

I can kind of make logical sense of a stand-alone pressurisation issue, but what really got my curiosity going was the major power fluctuations. At the top of the initial level off, we had several cycles of full power to full idle. And again when we re-levelled off at FL280. Whatever was wrong on the flight deck, the crew were working really hard to stabilise a power setting on one of the engines.

And the second big question I have is regarding the out of sync vibration. I am guessing that we flew most of the flight on mis-matched engine setting - anything to minimise the vibration we could feel in the cabin. But which didn't solve the real problem because after 15-20 minutes the out of sync would return, with more power adjustments to try and correct it again.

We had three issues going on, pressurisation, mis-matched engine power, out of sync vibration. I think the cockpit door being open for a few minutes at a time was a simple oversight, or maybe a lot of face-to-face discussion with the senior cabin staff about what was happening back in the cabin maybe?

CaptHaddock

may be HP bleed valve fault on one of the engines?

gb346

@ temp spike : The armchair enthusiasts at it again - deciphering the great red light mysteries from seat 7C and declaring the crew are incompetent idiots.

SAA has some of the best trained pilots in the world and they would have assessed the situation and tried to correct it according to the QRH. Sometimes there are failures which occur which are very uncommon and this is when experience comes to the fore and luckily, SAA has some of the most experienced crews on the continent. During the assessment, they would weigh up the severity of the failure/malfunction and decide to continue to destination, return to the departure field or carry out an en-route diversion. Part of their procedures includes a briefing to the cabin crew face to face and this is when the cockpit door was probably opened. As for worrying about who closed the door, I think that was the last thing the pilots would be overly concerned about. Nothing stupid about getting the aircraft home in one piece IMHO.

As for the original question, the B737 uses bleed air for pressurisation. The bleed air comes from the engines so if one engine is malfunctioning and the thrust is all over the place then the pressurisation would become a bit erratic too. Haven't flown the -800 in years so can't be more specific about the likely cause but thats my 2c worth.

eagleflier

On the classics, yes the gear warning lights will be on.
On d -800, even with idle thrust and gear not down, the gear warning lights will remain off.

JG1

SAA also has a whole herd of PDI's sitting in the cockpits whose experience levels are certainly not the highest on the continent. But their ego size makes up for it.

The red light you saw could have been the cabin pressure warning if the aircraft was fitted with such a light as some are. If not the crew rely on an aural warning for cabin pressure which is the same sound as the takeoff warning and would have been cancelled had it sounded.

If the crew (and the rest of you) were not on oxygen it could not have been serious cabin pressure loss.

If they descended to 25000 (instead of 28000) it probably meant they had lost an aircon pack and prior to shutting down that pack it could have caused some vibration.

The thrust changes to the engine might have been the crew trying to unblock a stuck valve, HP or even anti-ice valve. If there were problems with the engine actually running it would more than likely have been shut down.

Shrike200

This time I actually payed attention, and you're sort of correct - actually on the classics, they also stay off (maybe some do, some don't who know's, it might have been yet another Boeing 'option', the one's I fly don't come on though). But it's the good 'ol -200's (the real classics!) where they do actually come on in the descent

I learn (remember) something new (old) every day.