Please change the 737 Pressurisation System
 

In 17 years on type, I have witnessed 6 O2 mask drops in the airlines I have worked for, all pilot error. Helios passengers have paid the ultimate price, but the problem is that despite more rigourous checks now being made by the flight crew, the worlds 737 'time on type' drops like a stone (especially in Europe), this WILL happen again. This system is the achilles heal of the a/c and I would like Boeing to change to a more reliable/automated system. Please give me an indication on this thread if you think Boeing should change the system before more people are killed by this totally unintuitive design. Maybe you could post how many times you have seen or heard of a 737 dropping the masks thru pilot error. The cost of changing this system will be far cheaper than what will happen if their is another accident!

7 yrs on the 737 : only one mask deployment error on all fleet ,due to missinterpreting of the window damage check by the crew. The check was changed soon afterwards by Boeing.
The error you're afraid of is not in the system ,it's the failure to comply with the SOP. Automation has safety issues too, you may read about this on numerous safety articles.

Nothing wrong with the 737 pressurisation system as such. There is however a problem with the Boeing checklist policy for the after take off checklist which states that the PNF challenges and responds to his own actions. In other words the PF is not required to verbally answer the after take off checklist and in the simulator I have frequently observed the PF being so engrossed in the flying the aircraft either manually or by autopilot, that he rarely looks up to double check the actions of the PNF.

This frequently leads to an inadvertent mis-switching of the bleeds and/or pack switches going unnoticed by the PF who hears the PNF reading out the after take off checklist challenge AND response and assumes because the response is correct that the appropriate switches and pressurisation instruments must be correct.

Boeing states that the after take off checklist should be called after the flaps are retracted. This allows an early check of the pressurisation of the cabin IF that check is done correctly by the PNF. However, it is common for airlines to add more checks to the after take off checklist. These may include such minor items as seat belts and landing lights off at 10,000 ft. Thus the after take off scan and checklist is not accomplished until much later than Boeing recommend and a defect or mis-switching of the pressurisation system can go unnoticed until too late.

The Boeing after take off checklist does not specifically mention that the cabin pressure instruments should be included. It only mentions bleed and pack and isolation switch position. Of course a wise pilot knows that the cabin pressure and rate of climb is a vital key in the checking of the pressurisation. But there are pilots that stick rigidly to the words of a checklist and fail to think outside the square. In other words unless it is laid down in checklist form the pressurisation instruments must also be checked as well as switch position (in the after take off checklist and scans), some pilots simply don't check these instruments.

I can sympathise with Boeing who assume that people that fly their 737 are properly trained when in fact there are those who rely blindly on a checklist to tell them what to do. That said, I consider Boeing should review the after take off checklist philosophy and add a specific pressurisation instrument check. Boeing should also change the checklist to have the PF respond to the PNF challenge and not to have the PNF challenge and respond to his own actions. That would go a long way to minimising the risk of missing of vital information relating to pressurisation.

Finally, consideration should be given to making it SOP that the pressurisation progress is confirmed every 5000 ft in climb and descent. This would decrease blind reliance on checklists to initiate what are really airmanship items.

I don't know why they dont just fix it so that in a non-normal pressurisation set up, you get auto-fail induced so it triggers a Master Caution. i.e set the logic so that passing 5000 for example, if packs are off or bleeds are off it trips. If it is intentional for some reason, you just cancel it but at least it makes the crew aware either way. I reckon that this would only be a software change with minimal sop change. But then im no engineer. I just drive.

Of course that only works on the digital ones.

The 737 has a manual method of ensuring the bleed system is connected to the pressurisation system and as such it will always be a weakness. It has also been recommended to Boeing that they include a visual warning of high cabin altitude and not rely on an aural warning which is the same as the take-off config warning.

Unfortunately the 737 is still reliant on operating philosophies that assume a certain level of experience and training which is no longer a valid assumption. To highlight the change in operating philosophy, look at the B777 QRH which has been designed to reduce as far as possible, crew actions.

Alexban- The incident I think you are referring to involved two elements that led to the masks dropping. The incorrect numbering of the cracked windscreen was the first element; not understanding that setting the cabin altitude to 13000' would lead to the cabin altitude warning horn, was the second.

My airline operates approx 70.000 sectors a year with the 737 (both 3/400 and NG aircraft).
Last year we spent a briefing item on bleed/press system due to Helios, but also because of the high amount cabin press warning in the first half of 2006.
We had 10!!! cab pres warnings in the first 6 months.
that is 1 in 3500 flights. A far to high number considering this is the last line of defence against loss of cabin pressure.
But in my humble opinion, I find it absolutely rediculous that boeing is allowed to still produce an aircraft with a 1960's design which I find an ergonomical nightmare to operate.

Spuis

I am very surprised at this thread. I have no negative issues with the system (12 years on type & multi legs per day.) We use the counter-clockwise C to configure for bleeds-off takeoff and clockwise C after takeoff to configure to bleeds on. I don't understand why there is any difficulty operating such a simple system.

There have been, and still are, enough crew mistakes & incidents with the 737 pressurisation system to say that it is ergonomically substandard. The system is over 40 years old – almost half the lifetime of aviation - and was one of the first to be designed for a crew without a flight engineer. The world has moved on and the science of human factors has helped us identify & make many improvements in cockpit design, the 737 pressurisation panel should follow suit.

If I had any input I would suggest the following to Boeing in order of priority:
1. As TM said above, change all pressurisation items in the checklists to be challenge & response.
2. Change the cabin altitude warning horn to a verbal warning, say “CABIN ALTITUDE” or similar. The current horn is a hangover from when the only warning sounds came from the aural warning module. We now have a remote electronics unit which has voice capability, eg “PULL UP” etc.
3. Change the MAN caption to amber so it will illuminate on recall. That way no crew can accidentally get airborne in manual mode. If you are intentionally in manual mode I don’t see a problem with having an amber light. A lot of other things must be u/s to fly in manual.
4. Separate the diff pressure & cabin altitude gauges, they can be confusing in a non-normal situation (trust me, I have seen a few bizarre ones on airtests!), particularly when they are moving in different directions.
5. Enlarge the gap between +4000fpm & -4000fpm which is presently about 15deg on a small gauge. It is difficult to tell which it is when at max deflection. Yes I have experienced both!
6. Make the manual open/close toggle switch a selectable position switch, ie you set a valve position rather than more open or closed.

• Boeing has changed this panel before from CPCS to DCPCS, they can do it again.
• M/C logic has changed before (on the ctr tank pumps), they can do it again.
• The checklists have changed before (no comment!!!), they can do it again.

None of the above suggestions, or others better than these, are impossible to implement. Why the resistance?

Panel photos here for those not on type.

S&L

Thank you ladies & gentlemen for your input. Like watching the apprentice, I find some of these ideas are simply reinventing the same problems for the future. If you have 12 yrs on type then I believe you would find it difficult to mess up the configuration and misinterpret the guages. What we need is a completely NEW system. Surley saving 300kgs on a bleeds OFF takeoff is a complete waste of time. Surely technology could turn bleeds OFF as takeoff power is set and thereafter at 1500ft back ON again! Surley PILOTS dont need to get involved in playing around with switches during a high workload period, for 300kgs!
Lets all face it, history repeats itself; the 737 pressurisation system has not changed; pilots still mess up checklists for numerous reasons; the system (pressurisation) is not well understood during early transition onto complex types by new crews. Boeing needs to smell the coffee, spend some money and reduce the risk!

Ok. Airmanship 101:

If you hear a horn when you advance the thrust levers it is a T/O warning.
If you hear the horn in flight and you can cancel it with the ALT HORN CUTOUT button it is a pressurization warning!!!!!!!
If the letters under the selector say MAN and the blue light says MANUAL you are in MANUAL Mode.
If the letters under the bleed switch position say OFF then the bleed is OFF.
If the letters under the bleed switch position say ON then the bleed is ON.
If your ears sense something is unusual that is a cue to check the system.

If this is a problem for intrepid aviatiors then how on earth will they handle a fuel leak or hydraulic failure or even remember to retract the flaps after takeoff?

Tree-What seems obvious when written on paper is not always so obvious in day to day operations. In theory SOPs and following checklists should prevent any aircraft from being operated with the bleeds off above 10,000' and the warning horn being recognised for a high cabin alt; but time and time again, unfortunately sometimes with fatal results, the obvious doesn't happen. With many variables on the day an error can and will occur.

As Captain Sandl said its an old system that needs to be updated. At a minimum the aural alert should be backed up with a visual alert, which has been recommended to Boeing by several aviation safety bureaus.

Wrong, happens too fast.

What's the problem...
 

Gentlemen,

It's been many years since I flew the 737, but after over 17 years on the aircraft and over 12,000 in the left seat, I never had a problem with the pressurization system. The design is pretty much simple and foolproof.

Now, having said that, I've not flown the NG 737, so I am not familiar with any changes that may have come along. I question those who have had problems: What, specifically, has been happening that caused the problems?

As many have stated in this post, if you follow the procedures, the system pretty much is failsafe. (As we all know, nothing in aviation is foolproof and failsafe....but, having said that, the 737 is one of the most reliable and simple-to-operate commercial passenger aircraft ever made.)

Southwest Airlines is the largest and most experienced 737 operator in the world. As far as I know, they've not had any problems. After reading this initial posting, I've contacted my buddies at Southwest to ask them if they've had any problems. None has any information of any operational problems with the pressurization system.

So, again, what, specifically, has been causing all of this???


PantLoad

OK here are two incidents.

INCIDENT 1

Crew well rested. Capt flew VIP 747 in gulf. It was an ex Lufty a/c with the overhead switches reversed.

Crew missed the aural warning CB as it was pulled by engineering the night before doing auto speedbrake function tests.

Capt set the Air Cond Press panel up in reverse after start. Bleeds Off, Packs Off. FO read checklist but didnt check visually.

Bird strike on takeoff roll.

After takeoff checks performed poorly by FO who was more interested in the bird strike and whether it had gone thru ENG.

10000 ft checks performed poorly again with the usual pope like gesture at the air cond panel but didnt know what he was looking for I suspect.

No CAB ALT warning at 10000, CB was out remember.

Master caution overhead at 14000ft. Crew observed O2 drop, but didnt believe it. Called CCM in who confirmed it.

Normal descent initiated, normal landing, although Packs and Bleeds reconfigured on short finals.

This happened 6 years before Helios' accident.

INCIDENT 2

Capt elected to fly a bleeds off takeoff for practice. Not on a training flight either.

A/C began to pressurise after takeoff thru the APU.

After flaps up, FO started to reconfigure whilst the Capt hand flew.

Capt watched the FO do the reverse C flow, bleeds ON etc, but thinking job done, didnt see FO turn OFF the PACKS.

A/C depressurised slowly and CABIN ALT warning went off as a/c climbed thru FL230.

The crew thought it was a CONFIG warning and started looking for CB to pull.

then they woke up and went down.

Both these crews were highly experienced average operators. Yes of course it doesnt look like that reading the facts. But I do know there are many many weaker operators than these 2 crews out there.

So, there you go Boeing, Helios part 2 and part 3, almost! :ugh:

No comment...
 

No comment...

There have been several cases of crew incapacitation with fatal results due to mishandling of the pressurization systems. One was a bizjet (can't confirm the type) departing Europe and terminated in the North Atlantic. Another was a Learjet with a professional golfer onboard (USA operation) and another was a BE200 in Australia. In the last 2 cases the wreckage was available and the bleed switches were found in the off position. I and thousands of other pilots have operated these aircraft types without incident over hundreds of thousands of hours.

I say again: Airmanship 101

The 737 press system has been noted as being deficient in ergonomic terms. It has caught operators out at a few airlines. The accident report into the Helios Crash makes note of this. Here are a selection from 2 operators. The Aer Lingus incident was particularly serious. Please Note that these happened across most variants: -200,-400,-800.

Aer Lingus:

http://www.aaiu.ie/upload/general/3504-0.pdf

FR:

http://www.aaiu.ie/upload/general/3496-0.pdf

http://www.aaiu.ie/upload/general/3893-0.pdf

http://www.aaiu.ie/upload/general/6693-0.pdf

I think there was more, but you get the picture.

So ,as you say Jambo, as the pilots go dumber and dumber the plane should go smarter and smarter. You are very correct, that is why they invented the Bus..:} ....don't shoot guys..:E

Not as easy on the Classics, but it seems to me there is plenty of room on the NG EICAS display to get detailed Pressurization system info. The NG is unique in that the EICAS has very little to do with Crew Alerting (with the exception of specific engine warnings, everything else is through the master caution and annunciator panels), I don't see why it couldn't be reprogrammed to add non-normal config advisories/warnings as with the rest of the Boeing line.

Alexban, yes I suppose airmanship, common sense and experience are ALL in serious decline. I always thought I was getting into a state of the art industry with top notch jockeys. I was wrong on both counts and have had to accept that it is all pretty average really. Now that I have accepted that we are all average, the 737 is too complicated in the Air Cond and Press dept. Several serious mistakes have been made, but the system goes on. The master caution system is made by Radio Shack (joke) and is also a seriously poor ergonomic excuse for a warning system!