TUI Pressurization Failure Final Report

[Evan]

https://assets.publishing.service.gov.uk/media/67335df9bac74dd6026bf2dd/Boeing_737-8K5_G-TAWD_11-24.pdf

Interesting. Almost Helios all over again. Only the cabin altitude alarm stood in the way. That was the only slice remaining.

[bstolle]

Helios was almost 20 years ago so that's not bad at all from a safety POV.

[Gabriel]

The cabin altitude alarm stood in the way in Helios too. Or tried really hard to. It kept sounding all the way but was ignored.

[bstolle]

Can't remember because it's way too long ago, but AFAIK there's no such thing like a dedicated cabin altitude alarm or message on the old 737s.
There's only the Master Warning and then you have to check every single switch/light in the stone age cockpit to find out why on earth it is on.

[Gabriel]

Can't remember because it's way too long ago, but AFAIK there's no such thing like a dedicated cabin altitude alarm or message on the old 737s.
There's only the Master Warning and then you have to check every single switch/light in the stone age cockpit to find out why on earth it is on.

You are partially correct. There is no 100% dedicated warning, but close. The take off configuration warning and the cabin altitude warning share the same “toot, toot, toot…” warning sound, but the former is inhibited in the air and the latter is inhibited on the ground so there is never ambiguity as to what that warning sound represents.

[Evan]

Can't remember because it's way too long ago, but AFAIK there's no such thing like a dedicated cabin altitude alarm or message on the old 737s.
There's only the Master Warning and then you have to check every single switch/light in the stone age cockpit to find out why on earth it is on.

There is a RED dedicated CABIN ALTITUDE warning light on the forward panel, on the NG.

[Evan]

The cabin altitude alarm stood in the way in Helios too. Or tried really hard to. It kept sounding all the way but was ignored.

Yeah, this is my point. One ignorable slice of swiss cheese away from disaster. Hopefully, newer aircraft will have a better defense against this. For example, a take-off configuration warning**, that can be silenced on low-level ferry flights (I would suggest a guarded switch for silencing it, as it would be rarely used, but ATL will not tolerate any more guarded switches on this forum) .

**i.e at least one bleed air switch must be in the ON position when advancing the throttles for takeoff.

[ATLcrew]

**i.e at least one bleed air switch must be in the ON position when advancing the throttles for takeoff.

What if WE have to do a no-bleed takeoff?

[Evan]

What if WE have to do a no-bleed takeoff?

I want to say guarded switch but...

I'm thinking that little inconvenience might be justified by the safety it adds.

The reason I would want the warning at takeoff is that this is where the pilot error happens, in the pre-takeoff check.

[bstolle]

If you want to add a (guarded) switch for every checklist item which can be missed, you need a whole dedicated large switch panel in the cockpit.
Besides that, if you forget to turn on the bleed switches, what keeps you from forgetting the "bleeds-off-takeoff switch" as well?

[Evan]

If you want to add a (guarded) switch for every checklist item which can be missed, you need a whole dedicated large switch panel in the cockpit.

Yes, I’m sure that why I didn’t suggest anything of the sort.

[Evan]

Besides that, if you forget to turn on the bleed switches, what keeps you from forgetting the "bleeds-off-takeoff switch" as well?

For one thing, I think an open guarded switch is harder to miss in a scan of the overhead. But even if you miss that, there is still the cabin altitude warning. I’m just adding another layer of safety.

But, as guarded switches are forbidden on this forum, there is a better way. Make it a normal switch that resets when transitioning to air mode. The next crew that needs a no-bleeds takeoff will either have the switch on their checklist or will get a config warning and press the switch to silence it. And the odds of that switch being pressed in error (an active error) are far, far remote than bleed switches being missed altogether (a passive error).

[bstolle]

And the odds of that switch being pressed in error (an active error) are far, far remote than bleed switches being missed altogether (a passive error).

You think so? There have been numerous cases of pilots retracting the flaps instead of the gear after takeoff, extending the flaps instead of the speedbrakes etc.

[Evan]

You think so? There have been numerous cases of pilots retracting the flaps instead of the gear after takeoff, extending the flaps instead of the speedbrakes etc.

Yes, there have been rare cases of these errors. Compare that to all the cases of pilots forgetting or skipping or overlooking a checklist item and you'll see my point.

[Gabriel]

So many factors here... Not trying to make a point below, jut a random collection of random thoughts.

The pressurization system should be set up and checked before take off. That would normally include set the outflow valve to auto and set the target cruise altitude.

I believe that the bleeds must always be set to off initially, and that they are turned on (and checked) after the engine start up. If it's a bleedless take off I think that you still turn the bleeds on after engine start because you need the PACKs to condition the air during taxi, and the PACKs require bleed air, and then turn the bleeds off immediately before the take-off. In this case the bleeds need to be turned on at some point, possibly immediately before or after the reduction from take-off thrust to climb thrust (the need for a bleedless take off may be triggered by runway length, second segment climb or obstacle clearance, so it would be a good idea to wait until thrust reduction to standardize and to keep the cabin as "sterile" as possible during the initial climb which is a quite critical moment).

The plane should still "pressurize" even the bleeds off, (not increase its absolute internal pressure, but increase its differential pressure as the pressure outside goes down), as long as the outflow valve is closed or in "auto". That means that the cabin pressure should climb more slowly than the airplane itself. Now, by how much? That will depend on how well sealed is the plane (how large the leaks), which can be quite variable from plane to plane even within the same type. If the leaks are very small, the outflow valve may even remain a bit open to let the cabin achieve the standard cabin altitude climb rate. If the leaks are on the larger end of the spectrum, the plane with an outflow valve fully closed (without bleed air and PACKs) may still be too leaky as to hold a standard cabin altitude climb rate (the cabin altitude climb rate may be lower than the plane's climb rate, but higher than normal).

Considering all that, it may be a good idea to:
- Have a warning for excessive climb altitude (for example, if > 1000 fpm). That may be not very effective when the bleeds or PACKs are forgotten off but will be effective to catch an outflow valve left open or some other "big hole" preventing the plane from pressurizing.
- Have a warning for all bleeds off or all PACKs off that is armed only when the plane is in "clean" configuration.
- Have corresponding EICAS / ECAM messages for those with corresponding action items. That will of course not be implemented in a 737.

Now, seeing that in this age pilots are not donning their O2 masks after a cabin altitude warning is really concerning. One would think that we already learnt that lesson. But we didn't.