Take Pressurisation More Seriously


Dear old school mate,

Aircraft in flightGuess what! I've finally been put onto a pressurised aircraft! I've just had a chance to fly the company's Cessna 421: it looks the same size as a 402 or a Navajo, but it feels solid and heavy. It sounds truly awesome when it takes off. I'm really looking forward to being let loose on it, because it doesn't look that much more complicated.

Here the training is totally different. Bill (the ugly old check and trainer) spent two days with me doing a ground school on the 402 when I first got here and has done another whole day on the engines and pressurisation on the 421 since then. When I did the endorsement recently (it was similar to what I did with the 402) it wasn't rushed, and Bill made sure that I was OK with all the abnormal stuff and the instrument approaches as well.

Bill still doesn't let you go off by yourself (like we used to) but he flew with me on the job for a while. He called it line flying. I wasn't quite sure why at first but he just sat there and watched. When we were up in the cruise he'd ask me questions, but on descent when it got a bit busy he'd just sit and watch. If I stuffed it up a bit he'd talk about it after all the passengers had gone and ask me about how I had controlled my descent profile. It was good really, because he helped me to work out a system that would work better.

Having said all that, the 421 was a different kettle of fish. Flying it wasn't actually that difficult, but the numbers were all different. The engines are much bigger, and all the power settings, fuel flows, and speeds aren't the same. Descending faster meant that the system I'd worked out for the 402 didn't work on this one. I finally got that sorted out, but Bill kept on going on about the pressurisation. I didn't quite get what he was on about during the endorsement flying, but when we were doing the line flying, he was continuously asking questions about why the pressurisation could get me into trouble.

The problem (so Bill says) is that if the pressurisation isn't working, then the vital bits of your body start to conk out first (Bill has a way of saying things). The bit of your body that's going to save you is your brain, but that's what stops working first. When you start to run out of oxygen you lose the ability to recognise that something has gone wrong. He says it doesn't matter what you can see in the cockpit, if your brain can't work it out this means you've got a problem.

He then told me to go away and think about what would tell me if the pressurisation wasn't working OK. I did, and I thought that my ears would be the easiest: if they feel like I'm climbing in an unpressurised aircraft then something is wrong with the pressurisation. Fine, Bill said, but you can't rely on the feeling in your ears as you might not notice it every time. If that's the case, then you can't trust your life to noticing any change in your ears. If your head is clear, your ears equalise much more easily than if you've had a cold in the last few weeks. Also, since I'll still be flying an unpressurised aircraft a lot, I'll be used to that sensation, and I'd be less likely to notice that it's different in the pressurised aircraft.

The best bet then, he reckons, is the pressurisation gauges. The 'cabin pressure differential' gauge should be reading something more than zero, and the cabin altitude must be well below the real altitude on the way up. Also the cabin Vertical Speed Indicator should be reading a lot less than the main VSI when the aircraft is established in a climb. The cabin altitude is what really matters, so why not watch that?

Bill said that if you look at the 'cabin diff' and the VSI as well, and they are both reading something sensible, then you aren't trusting your life to only one instrument, and you are now in a reliable condition as there is 'system redundancy'. This means that you have more than one instrument telling you the same thing (that the pressurisation is working) and if one instrument wasn't working then they wouldn't all be telling you the same story and you would know that something was wrong. (This is a bit like improving reliability by having two of anything else, Bill says.)

This is fine, Bill said, as I now have a reliable set of instruments telling me that the pressurisation is working, so the least reliable bit of the system must be me! I've got a commercial licence, so I must be a professional pilot! How can he say that? If I'm not reliable, then why did he give me this job? For once he didn't get cranky. He pointed out that everyone expects me to do a good job, but since missing the pressurisation just once in my flying career is one time too many, it's his job to do everything he can to help me make sure that I never miss checking the pressurisation is OK. Well I didn't argue with that, and I listened more. Bill said that he thinks that the only reliable indication is the gauges, as I couldn't guarantee I'd notice something different in my ears or that I'd see the cabin altitude warning light.

Bill said that since the pressurisation was set to kick in at 500ft after take off, he required all his pilots to check the pressurisation gauges before giving a departure broadcast and then again passing 10,000ft before setting the altimeter for the flight levels. This way if I missed one check, I should catch the next one. Bill also requires the cabin altitude at 10,000ft to be written down on the flight log as well as the maximum cabin differential when we are up in the cruise. I'd never really understood why he wanted that but he pointed out that he had this done so that he, as my manager, would know that I had done this safety critical check and that's the only reason that I have to do it. Bill does all this to try and make me (the most unreliable bit of the pressurisation system) as reliable as he can. It sounds a bit odd to have him think of me as just a part of a mechanical system and using this 'programming' to improve me as a part of that system!

OK, so what about the cabin altitude warning light? It should light up at a cabin altitude of 10,000ft, which sounds OK as no-one worries about flying at 10,000ft without oxygen. Bill is very rude about this. He points out that this is the very last chance you get if you miss out on noticing that the pressurisation is set wrong and that this last chance to save you is just one little light that isn't even in your line of sight where you're normally looking.

Instrument panel of a Cessna 421 Golden Eagle showing the position of the cabin altitude warning light (left of picture, shown in red). Below it are the cabin altitude and cabin differential indicator and the cabin VSI gauges.

The problem, he says, is that your eyesight starts to go when your brain does. So what? I asked. Well, he said, when your eyes do start to go, your eyesight fails first around the edge of your visual field (where you aren't paying much attention anyway) and the warning light isn't in the centre of your normal visual field. Not only that, but since this is your last chance, there isn't anything else to alert you that something is wrong.

He wouldn't be so worried if there was something noisy that got your attention at the same time as the warning light, as you would notice the sound and it would alert you that there's a warning light somewhere that needs something doing about it. Once you start to go hypoxic, and the vision and thinking ability start to fade, not only will you be less likely to see the warning light, and even if you do, there's a fair chance that your brain won't register that there's a problem.

On the climb, Bill reckons that there's only a limited amount of time for the warning light to be any good at warning you, as the cabin altitude will be increasing all the time. It won't be long before you'll never notice any warning light because you will have 'lost it' by then.

'So what?' asks Bill. Well, you mustn't trust the cabin altitude warning light to keep you safe, because you can't guarantee that you will see it, or realise what the problem is. So you need a system that will tolerate you making a mistake before you get to a situation where you might need to rely on the warning light. This means that you have to know that the pressurisation is OK well before the cabin altitude warning light might illuminate. It also means that if the warning does light up, and I notice it, then all the systems that Bill has put in place have failed and I am in a dangerous situation. Well, not this time, maybe, because I can do something about it, because I've seen the warning light and fixed the problem. But what the light has really shown is that the reliability designed in the procedures that Bill is beating into me, has failed. Bill said that he wants to know EVERY time I see that light, because he wants to make sure that I never do see it in anger. That sounds fine by me!

I guess that Bill must have had a scare with pressurisation some time ago to make him so cranky about it today, but I think that I've been lucky in having him explain the practical side of it all to me. There's no way that I would have taken it so seriously otherwise. I wonder how many other pilots get on when they haven't had someone like Bill bending their ear. I wonder if they understand how problems with pressurisation can lead to an accident without them even realising it. It's a bit spooky, because with most of the other things that I can think of that could go wrong, I'd at least know something about it, and I could try to do something about it.

That's not the case with a pressurisation problem!

P.S. Come to think of it, why don't these planes have a warning horn?

Type: Educational Fact Sheet
Author(s): Michael Watson
Publication date: 12 March 2001
Last update 07 April 2014