Mate, I Take Pressurisation More Seriously Now!
By Michael Watson
Dear old school mate,
Guess
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?