Boeing4ever
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727 Flight Condition Sensors

Wed Apr 05, 2006 2:05 am

I recently took a group tour of the local airport and now have questions related to some of the items I saw.

First, I know that the 727 gathers temperature data from the Total Air Temperature probe (TAT). I'm confused though as to how this probe works and if the temperature indicated by the TAT is the actual temperature.

The next question comes from the 727's Pitot-Static system and how the aircraft measures pressure (and what kinds of pressure) and airspeed. I know most of these sensors are located near the nose of the aircraft, but are there anymore located elsewhere that I may have missed on the tour?

And I guess as a side question, rather odd to stick it in this topic, but nonetheless related, how does a Cessna 152 measure temperature? This one the tour guide never explained, though he did show off the neat air conditioning system for that bird. Big grin

As you probably figured, the questions above are related to school work, but the tour and online research have now got me started. How similar are the 727's Flight Sensors to that of the 707 or the 737? What kind of advances have there been in such sensors since the 727 and Cessna 152?

Thanks for your help!

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SlamClick
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RE: 727 Flight Condition Sensors

Wed Apr 05, 2006 2:45 am

The 727 and most other airliners sense temperature in some manner then use mach number to derive TAT or SAT arithmetically.

The Cessna singles I've flown (and it has been 26 years) just had an OAT probe in the wing root and their mach number was so low as to not need to deal with TAT. They were probably down in the "wind chill" area rather than up in the "ram rise" regime.

The relationship on temperatures for jets is like this:

If you had a tower 35000' tall and a thermometer at the top, you'd read the temperature of the air around there. We might call this OAT for outside air temperature.

A jet is jamming through that air at 3/4 the speed of sound or more. Its passage heats the air locally. That is called ram rise. The ambient temperature of the air in which it is flying is called SAT or Static Air Temperature (perhaps because "outside" is not specific enough) SAT plus ram rise equals TAT or Total Air Temperature.

On modern flight instrument systems and flight management computers these are almost a gee-whiz number. All the calculations are pretty much done for the pilots by the computers. On earlier jets we had to be sure to enter the performance charts with the correct value - SAT or TAT to get anywhere near the right answer. The difference might well be 30oC or so.

The pressure-sensing ports are of two basic types. One is "Static" pressure which translates to altitude once the reference setting is known for the altimeters. ("Altimeter setting" in the Kollsman window) A rate of change of that pressure gives you rate of climb or descent.

Pitot pressure is ram-air pressure and it is a function of your speed through the air. When it is compared (behind the scenes) with the static pressure from that system it reads out in "indicated" airspeed.

One effect of flying in the thin air at very high altitudes is that the less dense air produces (in effect) a falsely low airspeed reading. This then is corrected in an Air Data Computer and we get "True" airspeed.

"Ground speed" of course is a navigation function and a separate discussion. I'm tired of ground speed discussions so include me out on that one.
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Boeing4ever
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RE: 727 Flight Condition Sensors

Wed Apr 05, 2006 7:44 am

Thank you for the info SlamClick. Interesting stuff.

  B4e-Forever New Frontiers airplane 

[Edited 2006-04-05 00:44:42]
 
SFOMB67
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RE: 727 Flight Condition Sensors

Thu Apr 06, 2006 8:18 am

Quoting Boeing4ever (Thread starter):
I know most of these sensors are located near the nose of the aircraft, but are there anymore located elsewhere that I may have missed on the tour?

If I remember right, there are pitot tubes located on both sides of the vertical stabilizer. These are both connected to the feel computer which I think regulates hyd pressure to the stab jackscrew and elevator actuators. The faster the plane goes, the slower these controls operate.
Not as easy as originally perceived
 
fr8mech
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RE: 727 Flight Condition Sensors

Thu Apr 06, 2006 8:32 am

Quoting Sfomb67 (Reply 3):
These are both connected to the feel computer which I think regulates hyd pressure to the stab jackscrew and elevator actuators.

The elevator feel computer actually increases the amount of resistance the flight crew feel in the column as speed increases. It does not affect the rate directly. The elevator feel computer does not have any effect on the stabilzer trim system.
When seconds count...the police are minutes away.
 
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HAWK21M
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RE: 727 Flight Condition Sensors

Thu Apr 06, 2006 6:50 pm

Quoting Sfomb67 (Reply 3):
If I remember right, there are pitot tubes located on both sides of the vertical stabilizer

They provide Feel computer Pitot pressure.
regds
MEL
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