Another Use For Your Altimeter Watch
Apr 19, 2012 How to put your Altimeter watch to use in an
OK, admittedly this is geeky but I thought I'd share it anyway.
Sometimes while riding in the back of a 737 I'd like to know our
altitude. GPS works but it's finicky inside cabins. If you're lucky
enough to be be on one of our increasingly common (Southwest Airlines)
flights with web service, there's a cool (and free) web page that shows
where you are in the country, how high, and how fast you're going. No
more "where are we" questions. But there's that pesky requirement to
turn off your electronics just when curiosity peaks. So I use my my
ancient, but trusty, Suunto. Granted, it's missing its bezel and
has some dim segments but it still tells time, wakes me for those
oh-dark thirty push backs and, most uselessly, tells pressure altitude.
Since our cabin is pressurized you'd think that altitude wouldn't be
very useful. You'd be right UNLESS you know how the pressurization
system schedules cabin altitude. As it turns out, Boeings use a chart
that makes this conversion pretty easy. I don't know what the chart
looks like on other models, but I snapped a picture one of our 737-700's
(same as the 737-800). This chart will be slightly different on the -300
and -500 models since they can only go up to 37,000 feet (also called
Flight Level 370) as opposed to the -700's 41,000 foot maximum altitude.
During climb the pressurization system automatically pressurizes the
cabin in such a way that it feels a constant 700 fpm (or so) climb rate
regardless of the airplane's climb rate. If the airplane levels off, the
system stops the cabin's climb at the location shown by this chart. So
it won't be as accurate when the plane is climbing or descending.
Generally speaking, during climb, when your watch sees 2000', the
aircraft will be at approximately 16,000. At 4000' on your watch the
aircraft altitude hits 26,000' until cabin altitude (what your watch
reads) hits 8000' which would mean the aircraft is above 32,000 feet.
On descent the reverse is true. When your watch is reading the
landing altitude, you're probably only a few minutes away from landing.