Altitude Change : Newsletter Challenge (05/10/05)

Could the instruments have been shaken by the turbulence?

Bad weather=low pressure
Low pressure=higher (indicated) altitude

A low pressure air mass has moved into the area, and the weather has become unstable this would explain the clouds, turbulance and error in the altimiter.

Why would a low pressure air mass effect the Altimeter?

Altimere's are based on air pressure the higher you go the lighter (less dense)the air and the altimeter measures the air pressure and bases your altitude on the it's weight/density.

As noted in the description the weather has changed. This was accompanied by a change in air pressure which has affected the altimeter which works on a differential system. One port on the altimeter is open to the outside atmosphere and inside the altimeter is a sealed bulb connected by gears to a needle. Changes in altitude results in changes of air pressure entering the instrument compressing sealed bulb or allowing it to expand. This moves the needle via the gears. Unfortunately changes in atmospheric air pressure due to weather has the same effect. Which is why all aircraft atmospheric altimeters have barometric adjustment knobs and pilots pay close attention (hopefully) to weather and air traffic control reports of barometric pressure. If they don't "unintentional hull loss" can result. (An airline term for a crash)

"unintentional hull loss"??? Is there such a thing as intentional hull loss?
I guess that is why they also had to say there was no control tower in the question.

I'm not sure what the Term "unintentional hull loss" unless there refering to the unintentional loss of an entire aircraft "hull" and all. But usally they refer to the Hull as a fuselage when refering to aircraft

You're probably right. The aircraft industry is replete with euphemisms. My favorite is "near miss" for two planes that almost crash but don't. Shouldn't it be "near hit", since near miss means that you almost missed but didn't, i.e., a crash.

I first saw the phrase "unintentional hull loss" in an airline annual report. Their goal was to avoid using the word crash. It's right up there with "failure to validate" a phrase used to describe the launch explosion of a Arianne rocket.

I think the term "near miss" is used as opposed to a "far miss"...it's an indication that something nearly occurred. It stops the "well, no-one was hurt so why worry about it" response. While we're on the subject, when will "experts" (who should know better) acknowledge there's a difference between risk and hazard?

Well, I tried to post something immediately, but had to create an account first to avoid posting as an "Anonymous Coward" ;) anyway, now that I finally got a password, I'll try to be more timely...here's the answer: As all of us aging pilots know, things tend to "sag" over time...from the bright spring of youth to the cloudy, bumpy days that follow. Since you've been flying, you've been moving at a greater speed, and noticed this change more rapidly than others. Of course, the low pressure front that moved in and brought the clouds and turbulence also means that your altimeter, which depends upon a change in outside atmospheric pressure relative to it's internal bulb for it's dial position, is indicating to you an altitude that is inaccurate. Normally, as you fly higher the lower density of the air further above the surface, with lower barometric pressure, causes the altimeter to indicate a rise in altitude - and vice-versa as you descend. With a lowering of barometric pressure due to the front, the altimeter "thinks" that you are at a higher altitude than you really are. You need to call the regional flight service station, get the current pressure setting for your area, and "dial it in" so that the altimeter again reflects your actual altitude, keeping you from trimming the trees as you approach for a landing. ;) Blue side up!

As written in the 5/17 issue of Specs & Techs from GlobalSpec:

You have learned one of the reasons for the old flying adage "High to low, watch out below!" In the morning you were enjoying a nice high-pressure air mass. High pressure is usually accompanied by clear skies and calm air (at least near the center of the mass). As you were out having fun, a low-pressure air mass moved into the area accompanied by clouds and wind (which usually gives rise to a bumpy ride). Your altimeter is based on air pressure — the higher you fly, the lower the outside air pressure is which is what drives the altitude indicator. Since you didn't pick up a new altimeter setting, you were set up for a higher- pressure air mass than the current one you were in. In order to indicate 500 feet as you crossed the trees, you now had to fly lower than before in order for the air pressure to "push" the indicator to 500. Trouble! This is why one of the most frequently communicated pieces of information at control towers is the altimeter reading. Big plane, little plane, it doesn't matter — high to low, watch out below!