The Directional Indicator

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Whisky Tango
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The Directional Indicator

Post by Whisky Tango » Mon Oct 13, 2008 2:19 pm

As I'm making my way through my student navs I am noticing that the DI is very unreliable just after coming out of a turn, say onto the next leg of the trip. It can be 40 or 50° off the real heading, requiring a good bit of realigning every time.

Is this the case for all aircrafts? I can't imagine an airliner having the same problem every time they're given a new radar vector, so in what way are airliner DI's different to my trainer aircraft?

Papa8
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Post by Papa8 » Mon Oct 13, 2008 2:43 pm

My two cents worth (student also):

The drift is subject to many factors, the speed at which the earth turns, aircraft movement, little inprecisions in the gyro's balance which lead us as pilots to require a 10-15 min check of the DI (or DG or HI). There is a very thorough article on wikipedia here:

http://en.wikipedia.org/wiki/Heading_indicator

However airliners (and high-end light aircraft) long ago reduced this error-prone instrument by slaving it to a compass and came up with the HSI or Horizontal Situation Indicator. There are also slaved DIs in some light aircraft which predict drift through a latitude nut.
It only takes two things to fly:--- airspeed and money.

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Post by zulu10 » Mon Oct 13, 2008 8:20 pm

"A Flux Valve is a magnetic sensory device that electrically transmits information to an aircraft's compass system regarding its orientation relative to Earth’s magnetic field. This information, in the form of three separate electrical signals tied to a common return, is used to generate a visual indication to the flight crew regarding the heading of the aircraft relative to magnetic north."

This is what you are refering to. Does away with vacuums and gyros errors thank God.

ifty
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Post by ifty » Mon Oct 13, 2008 9:19 pm

zulu10 wrote:"A Flux Valve is a magnetic sensory device that electrically transmits information to an aircraft's compass system regarding its orientation relative to Earth’s magnetic field. This information, in the form of three separate electrical signals tied to a common return, is used to generate a visual indication to the flight crew regarding the heading of the aircraft relative to magnetic north."

This is what you are refering to. Does away with vacuums and gyros errors thank God.


A flux valve......sounds like it was missed out from Back to the Future :wink: :lol:
You pass this way but once, there is no such thing as normal. There is you and the rest, now and forever. Do as you damn well please or you could end up being a pot-bellied, hairless boring old fart.

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Post by stovepipe » Mon Oct 13, 2008 11:39 pm

A DI that has that degree of error after just one turn is basically junk.You shouldn't need to be correcting the error except for the 15-minute check.Listen to it next time after you have shut down and switched off.If you hear a rumbling noise instead of a smooth "electrical" noise as the gyro winds down, then it's scrap.Tell your instructor to put it on the snag sheet and make it a point not to fly that aircraft again until the DI is changed.
regards
Stovepipe

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Post by dara » Tue Oct 14, 2008 7:46 am

These days the airliners only use the alaved gyros for the back ups, the main displays work off the Interial Reference Systems.
And agreed with stovepipe a degree out after every turn, good luck to the poor sod who ends up in the soup with that plane!
If you are asking me if this old plane is safe to fly, how do you think it got to be this old?

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Post by Papa8 » Tue Oct 14, 2008 11:30 am

Flux Valve, yes that's it. Actually a little off topic but did anyone see that program on Discovery that showed the original inertial reference system in an aircraft It was a huge thing that required a person to make constant calculations based on a printout but they managed to fly it with reasonable accuracy across USA. It formed the basis for the design of the navigation computer that was used in the Apollo missions.
It only takes two things to fly:--- airspeed and money.

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Post by Dutch Roll » Tue Oct 14, 2008 5:59 pm

the inertial navigation system(INS) is the old type,ive seen that on the older videos,mad stuff compared to todays IRS which has ring-laser gyros (3 i think) on a strapdown system measuring angular velocity and a whole lot of maths with prisms etc!! highly accurate,little or no calibration required
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Post by Bob » Tue Oct 14, 2008 8:35 pm

A flux valve......sounds like it was missed out from Back to the Future


Old technology all right. Did a compass swing for a 727 a while back with flux valves.

There was a bit of head scratching for a while untill we realised that the left flux valve was over a nice big metal drain. :oops:

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Post by shamrock/heavy » Tue Oct 14, 2008 10:26 pm

I can only speak for the 738 which has ADIRU's Air Data Inertial Reference Units. The system consists of two independent IRS's (L+R), each of which has three sets of laser gyro's and accelerometers. Once aligned and in NAV mode the provide a hell of a lot in combination with the Air Data part of the system such as true an magnetic heading (stored mag variation in database), attitude, vertical speed, groundspeed, track, present position, and wind data to the aircraft systems requiring such info for example the DEU's Display Elecronic Units are fed from the IRS's these supply the 6 DU's on board with their info. Another system the IRS provides for off the top of my head is the EEC's Electronic Engine Control which via the HMU's meters fuel to the engine and provides N1 and N2 protection.

All in all quite an important bit of kit, kind of takes the fun out of flying though hey :wink: But at the same time never came across a DI that drifted 40-50 degrees off heading after a turn, sounds like the gimbals on the gyro are knackered.

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Post by Whisky Tango » Wed Oct 15, 2008 1:29 pm

Seems it was a problem with the ADIRU that made that QANTAS A330 take a nosedive last week!

http://www.atsb.gov.au/newsroom/2008/re ... 08_43.aspx

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