Attitude Reference Unit

Fri Aug 20, 1999 5:34 am

This device which is referred to as the attitude reference unit measures the aircraft's linear and rotational motion, inertial altitude, angular rates and vertical speed. They are then sent in to the secondary air data unit to be referenced according to the readings given. This is technically a 10 MCU device and its advantage is that it gets rid of the electromechanical standby horizon instrument and is replaced by modern sensing devices and of course the use of digital electronics.

What I want to know is when this device is implemented in wide-bodies, does it also require the use of an air data module embebbed in the main box, or is it sufficient to obtain measurements without it. Thanx in advance.

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Joined: Fri Mar 16, 2007 1:29 pm

RE: Ahrs

Fri Aug 20, 1999 7:11 am

I'm a pilot, not an engineer, so please pardon the layman's language -

AHRS (Attitude & Heading Reference Systems) came into popular use in the 1970's. Initially, they were essentially a multi-use gyroscopic platform which could replace multiple gyros thus saving weight and maintenance.

As their use proliferated, and as laser technology developed, many of the heavy maintenance intensive gyros were replaced by ring-laser platforms. The initial ring lasers had three axes (looked like a triangle). As the aircraft moved about its three axes, phase shifting of the laser beams could be measured, then translated into attitude and heading information to the pilots, in a conventional sense - electro-mechanical instruments: the attitude indicator and the heading indicator. However, for navigation purposes, the INS still needed acceleration information, and to the best of my knowledge still required moving-parts accelerometers.

In the later manifestations of IRS (Inertial Reference System), the ring laser gyro has been modified somewhat so that the strapdown platform of lasers can provide not only Att & Hdg info from the measurable phase shifts, but also acceleration.

The other significant improvement that took place over the years in conjunction with the basic hardware mods was the replacement of electro-mechanical instrumentation in the cockpit by EFIS (Electronic Flight Instrument Displays).

Once the full advantages of EFIS technology was realized in the early 1980's, other systems were plugged into computers that would provide information to the pilots. Eventually, the requirement for the Flight Engineer was removed.

With regards your specific question about ADC inputs into "wide body" aircraft systems, with this system, size really doesn't matter! A B747-400 may have the same AHRS system as a Dash 8!

All AHRS units have ADC input, and a multitude of other inputs. Your local aviation bookstore should have a section on aircraft avionics that will give you more info in this regard.

If I've over-answered your question, my apologies. Otherwise, hope the above is of use.

Best Regards,


RE: Ahrs

Fri Aug 20, 1999 10:34 am

Buff--I have also observed that selected avionics equipment can also integrate this system(secondary reference unit) to measure 'extra' flight characteristics such as heading and even acceleration which you were talking about. I guess this is apart from the standard EFIS but can also detect and control the entire system. This actually replaces the triple-redundant inertial reference and dual-redundant air data systems of before with a single self-monitored system which if I am correct is composed of a Secondary reference unit embebbed with its data module.

As you referred to gyros, this device actually contains 6 gyros and accelerometers within 3 power supplies and digital data buses combined. Would you know if standard wide bodies integrate this system as part of the EFIS or is it a separate matter?

Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Ahrs

Fri Aug 20, 1999 11:24 am

"This actually replaces the
triple-redundant inertial reference and dual-redundant air data systems
of before with a single self-monitored system which if I am correct is
composed of a Secondary reference unit embebbed with its data module."

Leo-ERJ: I've copied a snip from your post. In short: not quite - read on...

If you think of EFIS as an end-use device rather than as a primary device, it should make the rest a little easier to understand. Avionics are the single most confusing installation in modern aircraft!

To the best of my knowledge, all large jet aircraft have a triple IRS (Inertial Reference System) installation. From these IRS units, the INS (Inertial Navigation System) functions. Most also have a flight management system and RNAV (Area Navigation - uses scanning DME's to triangulate position). From the IRS comes the attitude & heading information - many times even if the navigation part of the device is not working properly.

Also to the best of my knowledge, all large jet aircraft have at least two completely independent ADC's (Air Data Computer). As well, there is also at least one independent standby pitot-static system and attitude indicator.

Most other "vital" systems are duplicated or even triplicated. An example of another triple reduncancy on modern airliners, including as far as I know, all wide-body designs, is the hydraulic system that powers the primary flight controls.

All the EFIS does is integrate the information display for the pilots. The systems are still there in the background, but their "raw data" is not usually displayed unless selected by the pilots. An example would be the Captain's primary EHSI (Electronic Horizontal Situation Indicator). The Capt (and the F/O) have access top each other's heading information, but only one is displayed at any one time. If for instance the Capt's heading source fails, he/she can select the F/O's source instrumentation.

If I understand your second post above, the systems you speak of are still installed. The EFIS is simply another system that displays information selected by the pilots. It gives a better presentation than the old electro-mechanical dial instruments.

Hope that helps.

Best Regards,


RE: Ahrs

Sat Aug 21, 1999 1:25 am

What I meant to say in the part you pointed out from my post was that that actually replaces it with a Fault tolerant air data IRS. It is actually what you were referring to later on where the navigation part is not working properly. This device has a IDEAL 100% fault detection. I understand that the FT-air data IRS will actually maintain functional performance and dispatchability even after component failures in any of the six fault-containment areas. It is commonly referred as to FT/ADIRU. It has a six gyro/accelerometer configuration as opposed to others of today that contain 12. Anyway, thanks for the info.


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