View Full Version : Catching the Pressurisation problem before it hurts.

16th Oct 2005, 03:48
The Tech Log posts on the Helios B737 accident have covered so many pages that many interesting technical issues have been in danger of vanishing in a sea of argument and counter-argument.

There were two links in particular, that attracted my attention. Both were official reports on 737's that had climbed unpressurised after take off because through sloppy cockpit checks no one had noticed the packs were off.

The first officer ( his total flying experience was about 370 hours) in one of the incidents had covered the bleeds off procedure in his type rating course on the simulator, but had never seen a bleeds off procedure in the real aircraft.

Subsequently, when he was asked by the captain to reconfigure to bleeds on as part of the after take off procedure, he inadvertently turned both air-conditioning packs off. He then missed the error as he read the checklist and responded to his own challenge. The captain, busy on other matters, failed to double check that the new first officer had done his job properly.

Now I am not knocking the poor first officer - heaven knows I have seen the same confusion on re-configuring the switches on countless occasions, both in the real aircraft and in the simulator. In fact I have no doubt readers of this post will mutter "Been there - Done that" under their breath.

The weak link here is the design of the after take off check list where the first officer (or PNF whichever) having in theory conducted the required scans, then pulls out the checklist and reads and responds to his own actions. The captain hears his response and most times, without double checking, assumes that the checks are done.

With this accent on Areas of Responsibility so beloved of modern checklist philosophy, I have little doubt that errors of ommission such as described are common-place. Personally I prefer the old policy of one pilot reads and the other responds for all situations. After all lots of trees met their end in the writing of books and articles on how challenge and Response check lists are Good Things.

As a firm believer in Murphy's Law, I have long since learned in aviation one should never totally trust the pilot in the other seat even if he is your best mate. Always quietly and subtly double check that he has done his job because although a verbal response may be according to company SOP, his action may not have been. Do I hear more mutterings of "Been there - Seen that?"

Having got that little bit of bulsh..t off my chest, I would like to ask a question of the Boeing 737 technical experts reading this post.

In both the links referred to earlier, the air-conditioning packs were either inadvertently left off after engine start or switched off as part of re-configuring after take off.

Either way, the aircraft climbed unpressurised right from the word go. What has me tossed is in one case the cabin altitude warning sounded as the aircraft passed 10,000 ft unpressurised (as one one would expect). But in the second incident, the aircraft must have been partially pressurised because if I recall, the aircraft was around 20,000 ft when the cabin altitude warning sounded. In both cases the packs were off from take off yet the results were different.

In the 737-300 simulator that I flew, the aircraft maintained partial pressurisation in the climb to 20,000 ft with both packs off, and again, if I recall correctly, the cabin altitude did not reach 10,000 ft until the aircraft was around 20,000 ft.

Your comments would be appreciated.

16th Oct 2005, 08:23
Hi Centaurus,

As you point out, to assume is to make an ass of u and me.

Good SOP will prevent climbing unpressurized, as you check that in the after takeoff cklist, 10000ft, and reaching cruise altitude, at least.

I have seen various times people screwing up their check using the challenge - response method also.

I'm always amazed when I simulate setting the flaps at 30, leaving them at 15 for example, and getting a response "30, green light" from what I normally consider smart FOs...

(Yes I know, I shouldn't do that... but I guarantee that's a lesson they aren't going to forget soon, so they grew up quickly).

Chatting with a colleague after the Helios accident, I found out an incredible incident had already taken place in our Company, before I joined.
Amazingly, but not surprisingly, instead of being of public dominion for the sake of everybody, few people know about it...


During a turnaround, the mechanic turned both bleeds off to do some check of his own...
Well, to make it short, the Belgian captain managed to skip it through the Before start cklist, After take off cklist, the Cabin altitude warning sounding, the falling of the oxygen masks...!!!!

Can you believe that?????

When the FA came to the cockpit, they were still climbing happily.
The airplane reached about 24000ft!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

They made an emergency descent, ad landed at the departure airfield, still with both bleeds off!!!!!!!!!!!!!!!!!!!!!!!!!

I can't find the words... better, I do have them, but better I keep my mouth shut :mad:

16th Oct 2005, 08:57
Centaurus - to try to answer part 2 of your query - it sounds as if the second a/c was 'tight' as far as leaks go - the leakage rates vary significantly between hulls. If the rate is VERY low (which is unusual) then the hull could retain whatever pressure it had for a while hence getting to 20 before the cabin reached 10?

As far as part 1 goes, I have always assumed that 'airmanship' would dictate the other pilot double checks critical systems? I always get my F/O to confirm the press. panel settings when I make them and vice versa.

16th Oct 2005, 10:35

The accidental bleeds off flight, or any other pressurisation panel cock-up, is one of the issues that is addressed by the new Boeing Normal Procedures. The after take-off checks now require that you specifically check the bleeds and the packs. See http://www.b737.org.uk/nnp.htm for the full description.

Engine bleeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . On
Packs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AUTO
Landing gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UP and OFF
Flaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .UP, no lights

BTW when I was a brand new F/O on the 737 I once ferried an aircraft to an airfield about 100 miles away. We noticed that the engines were quite slow to start and put it in the tech log at the other end. We were describing what happened to the engineer when he gave us a withering look and said “Did you fly here all the way with the bleeds off?” – the switch position confirmed that we had. The engine bleeds switches were very rarely moved and I had missed the fact that they were both off in my cockpit preparation. Since the pressurisation panel checks were read and responded to by the person who set it up (me), it was never picked up. Good job it was not a longer flight where the cabin would have had to pressurise.

Re the aircraft maintaining some pressurisation without the packs being on, I agree with BOAC that it probably depends upon the leak rate of the individual aircraft. The simulator test that you did probably assumed a leak rate of 2000fpm (-500: 2250fpm) which is the Boeing limit. On airtests I have seen leak rates vary from 1500fpm to over 4000fpm on aircraft of a similar age.


16th Oct 2005, 12:53
LEM. Interesting reply and thanks. I cannot understand why Boeing persist with the policy of the same pilot reading and responding to his own words. To me it is a recipe for potential disaster. I understand from a colleague who flies LH seat A320 that all Airbus challenges (normal and non-normal) are answered by the other pilot thus ensuring the integrity of the challenge and response.

CaptainSandl. As you say, the revised Boeing after take off checklist should prevent inadvertent incorrect switch selection (if used as Boeing intended). The absence of an effective central flight safety data base on incidents such as pressurisation boo-boos means that pilots only hear a fraction of what stuff-ups go on.

Because there are bound to be inadvertent mis-selections such as those I described, as well as the Helios Airways accident, in my view simulator instructors should include training on the subject. Too often simulator sessions are pre-programmed "canned" exercises entirely predictable and accent on ticking the regulatory squares rather than having a good look at the lessons learned from accidents.

For example how often have you seen in the simulator a specific exercise aimed at training for GPWS pull-ups from say a descent with speed brakes out at 250-300 knots? Not too often, I'll bet.

16th Oct 2005, 17:24
I have a hard time believing that neither crewmember would feel the physical affect of diminishing air flow and loss of conditioned air in the cockpit when both packs are switched off. I know that I would instantly know when air flow in the cockpit is cut off.

Subsequently, when he was asked by the captain to reconfigure to bleeds on as part of the after take off procedure, he inadvertently turned both air-conditioning packs off. He then missed the error as he read the checklist and responded to his own challenge. The captain, busy on other matters, failed to double check that the new first officer had done his job properly

Needless to say, without fresh air flow in the tiny cockpit the air would get stale very quickly.

19th Oct 2005, 21:04
The point that I think everyone is missing by getting bogged down in esoteric discussions about procedures, is that the crew should notice the lack of pressurisation due to the pressure changes on their ears. I have experienced pressurisation problems four or five times over the years, most recently three weeks ago. We identified that the aircraft was not pressurising before we'd passed 1000' agl, without looking at any instruments....

Stu Bigzorst
19th Oct 2005, 21:35

Sometimes, but not always. In a leaky hull I too recognised the fault before 1000'. But in our Airbus ops, all take-offs are done PACKS OFF. And you can't tell (apart from a bleating ECAM).

The point is "read and do" is so very wrong in the Boeing philosophy. It always surprised me that they do it this way, since Boeing always consider the FO a fool.

The only way round it to make it a personal discipline to follow the other guy while he is reading and doing, and make sure you don't see what you expect to see (I saw this the other day: Flaps? "5, 5 Green Light", when there was an amber light).