View Full Version : Analog flight Instruments

14th May 2006, 22:45
I was wondering today about the analog flight instruments in aircraft such as the 742, DC-10, L1011 etc. Wheras modern aircraft such as the 737NG and 744 receive the data for the PFD and ND from the IRS's, were do older aircraft recieve the data from?

How is the standby AI powered, how are the captains and first officers AI's and HSI's powered? Do they come from the INS's or do they incorporate their own special gyro's? If so are they driven from the main/standby electrical systems or are they vacuum?

Old Smokey
14th May 2006, 23:11

For the older generation aircraft that you mention, Attitude information came from dedicated Vertical Gyros (VGs), for both the primary and standby attitude indications. In the event of a primary VG failure, attitude information was transferrable from the operative VG to the failed side.

When INS was added to these aircraft, the attitude information available from the INS was largely 'wasted', as, in most cases, the INS was an "add-on' feature. Aircraft like the A300 which appeared in the transition period between INS and IRS did have attitude information supplied directly from the INS gyros.

For all of the aircraft that I know, Standby Attitude was always electrical, right down from Main Supply, Essential supply, and to batteries as a last resort. I think that you'd have to visit some aviation museums to find a Jet Transport aircraft that had Vacuum supply to any attitude indication.

Interestingly, the latest models of the B777 have gone full circle for Standby Attitude indication, no longer using ADIRU and SAARU information, but having their own Vertical Gyro.:D


Old Smokey

15th May 2006, 10:29
Ah Ok thanks for the reply.
So do the VG's spin up as the main busses are powered or do they have to be aligned in the same way that an INS is?

Is the HSI a similar situation and do they have to be corrected every so while like when flying GA?

Thanks for your help!

Old Smokey
16th May 2006, 00:03
The VGs erect all by themselves when electrical power is available, no ON/OFF switches, only a Circuit Breaker.

The HSIs similarly erect and align without outside intervention, no periodic re-setting necessary. The facility exists to periodically re-align from the Magnetic Compass in the event of a Flux Valve failure (also the case with IRS).


Old Smokey

16th May 2006, 02:29
Old Smokey has certainly hit the nail on the head, and indeed when INS equipment (and FMS's as well) were added at the customers request on the L1011, the attitide information was hardwired directly from the respective INS unit.
In addition, complete rad/nav switching was available for both pilots, to select the operative equipment.
The standby horizon is powered from the DC standby bus which, if all other electrical systems failed, would be directly conected to the battery bus, supplied by the ships battery.
Some operators, especially those that operated long overwater routes, provided two ships batteries, for added redundancy.
In 26 years of operating the 'ole Lockheed tri-motor, the ONLY item in the electrical system that failed (very infrequently) was an engine driven intregrated drive generator, and when it did, it was disconnected from the engine by the Flight Engineer.
Lockheed.....works GOOD, lasts a loooong time.:ok:

Ah Lockheed!!:D

17th May 2006, 19:03
Thank you both or your replies.

I wish I still had a chance of flying one of the classics, one of the only things I'll be quite sad at not doing. Maybe one day I'll get a go in the sim...

17th May 2006, 19:17
Oh boy 727, that's long ago. As far as I remember it had an airdata-computer,:confused: WOW.

But the output was just TAS and SAT.

Please correct me if I'm wrong.

17th May 2006, 21:34
where do older aircraft receive the data from?
- for a little bit of interest - the BAE Lightning (yes, an older aircraft':)) had, in the Mks 3,5 and 6 (and the Saudi versions), a main AI which was driven from an inertial platform taken from the cancelled 'Blue Steel' stand-off cruise missile. No gyros in the instrument itself. The only 'navigation' type input from the platform was profile rate. Quite advanced for the time.

Old Smokey
18th May 2006, 01:41

Like you, it's a long time (7 conversions ago) since I handed back my B727 manuals, but I'm sure that there'e a lot of B727 pilots who cruise these pages and will correct both of us if we're wrong.

I recall that the standard B727 did not have TAS or SAT indication at all! The ADC did do the mathematics to provide a good conversion from RAT to displayed TAT (Ct = 0.99) on a giant indicator. That was the only indication 'on the surface' to pilots that an ADC was at work. Of course, behind the scenes, it was doing a great deal of work in the provision of Altitude, Airspeed, and Mach Number indications, and had a not inconsiderable input to the AFDS, Auto-Pilot, and HSI (the latter to provide indication of acceleration to reduce acceleration errors in Magnetic Indication). Pressurisation systems etc., also benefited.

TAS was always available from the ADC, but not utilised until the advent of latter day INS installation for wind calculations (at least in the Delco version).

Extant B727 operators, please correct me where I'm wrong! :ok:


Old Smokey