Aircraft Sensors, Shared?

[Obi Offiah]

I've been wondering, are the rate sensors, gyros and accelerometers used in aircraft INSs, independant from the sensors used for the flight control systems?.

Thanks
Obi

[Wodrick]

The original how long is a piece of string question. Broadly, and very generally :

Specifically INS rather than IRS.

If the Aircraft was fitted with INS from new then the answer is – Probably yes. The possibility exists that rather than a third INS for Standby purposes it might be a tray of Gyros instead and just be an Attitude/Heading source. I’ve seen both.

If the INS was retro-fitted then the answer would be probably no, it’s normally too much work to rewire all the equipment that need Attitude and Heading ( think about it ).

This is a VERY general answer, I now expect a plethora of responses “We retro fitted our fleet of Mk IV Scrungejets in 1975 and replaced all gyros” etc.

[Obi Offiah]

Thanks for the reply Wodrick. :)

I edited my initial post by changing 'using' to 'used'.

Is it typical of older aircraft to have the two sets of sensors? I ask because using a stabilised platform system for the INS and then an airframe referenced system for the FCS, should reduce the amount of processing necessary I would think.

Thanks
Obi

[Swedish Steve]

I sit here trying to think of a snappy answer, but there isnt one. Aircraft have evolved over time.
The B747-100 had INS which were big boxes of gyros. They only provided navigation. The ADIs were driven by their own gyros.
The Tristar had positions for three INS, but we only had one fitted on ours, and two racks each with a VG and a DG. The INS drove the ADI on one side, and the VG DG drove the other.
The B737-400 had TWO INS, each driving its own ADI, and able to feed both sides. There was no spare. There was of course a standby ADI.
The B777 has a single INS (with six gyros inside it) and a SAARU (standby unit with gyros. Either box can drive all three ADIs.
I am sure there is something better in the B787.
I would appreciate it if someone could correct me if I am wrong . I am an A and C guy who spends his life on the ramp.

[BOAC]

According to 'spannersatcx' (post#10, http://www.pprune.org/forums/showthread.php?t=240665 and 'Checkers at Post#13) there are also separate accelerometers for landing 'g' and other bits.

[Filler Dent]

Yes, seperate accelerometers are usually the FDR accelerometers and are also used for Aircraft Condition Monitoring Systems (ACMS). All sorts of data can be accessed if you know the right codes, ask an Avionics engineer.

Swedish - I don't wish to sound pedantic, but surely you mean IRS on a 737-400 and a B777. Totally different innards, although the outputs are almost the same I admit. :)

[ICT_SLB]

Obi,
Think there's a lot of confusion with some of the retrofit & older systems. On any aircraft post Boeing 757/767 - 737NG, & most Regionals the same Attitude/Heading system (whether AHRS or IRS boxes) provides identical but seperate databuses (generally ARINC 429HS) to both the Displays and the Flight Guidance/Control Computers - usually split Onside displays, onside FCC & Offside Display (for reversion). Pitch, Roll & Heading plus their appropriate rates & accelerations are all on the same bus. IRS also provide both body & flight path referenced data hence their use on HUDs and also Latitude & Longitude. INS is only a relatively simple FMS grafted onto an IRS to provide a navigational function - nowadays most aircraft fit a completely seperate FMS with much more functionality & using GPS for primary position.

All Part 121 & 91 aircraft now also have to carry an FDR system one element of which is a dedicated Triaxial Accelerometer mounted close to the aircraft center of gravity that is used to reconstruct the aircraft flight path in the case of an incident.

[Obi Offiah]

Cheers everyone, I really appreciate all your answers.

I've been trying to solve a couple of troubling questions surrounding flight control systems, that have been bugging me for a few years :ugh:. I've attempted to cover all bases which is why at the moment I'm focusing on the FCS sensors and their possible relationships to the INS/IRS/AHRS.

ICT_SLB:
Its absolutely critical that at the very least, longitudinal and lateral acceleration data are furnished to an INS for 2D navigation. I think most systems also provide vertical velocity via a normal accelerometer (stabilised platform)?
Regarding flight control systems, I believe the minimum of a lateral acceleration and normal acceleration sensor are essential, to grant the required level of fidelity demanded by a modern FCS.

Considering both the older stabilised platform sensors and the modern strapdown systems, how usual is it to find longitudinal acceleration data fed into the flight control computer to aid aircraft flight characteristics?

BTW Swedish Steve, the B777 gyros, are they ring laser gyros?

Thanks
Obi

[Swedish Steve]

BTW Swedish Steve, the B777 gyros, are they ring laser gyros

Yes. As far as I know all airliners from the B757 onwards are laser gyros.

[ICT_SLB]

Cheers everyone, I really appreciate all your answers.
I've been trying to solve a couple of troubling questions surrounding flight control systems, that have been bugging me for a few years :ugh:. I've attempted to cover all bases which is why at the moment I'm focusing on the FCS sensors and their possible relationships to the INS/IRS/AHRS.
ICT_SLB:
Its absolutely critical that at the very least, longitudinal and lateral acceleration data are furnished to an INS for 2D navigation. I think most systems also provide vertical velocity via a normal accelerometer (stabilised platform)?
Regarding flight control systems, I believe the minimum of a lateral acceleration and normal acceleration sensor are essential, to grant the required level of fidelity demanded by a modern FCS.
Considering both the older stabilised platform sensors and the modern strapdown systems, how usual is it to find longitudinal acceleration data fed into the flight control computer to aid aircraft flight characteristics?
BTW Swedish Steve, the B777 gyros, are they ring laser gyros?
Thanks
Obi

Obi,
Reread my post. All modern Attitude/Heading sources contain both a form of gyro [(AHRS - either rotating "Bender" (Rockwell-Collins) or micromachined sensor (Honeywell, Thales and several others - all using a Systron Donner assembly) or (IRS & INS - Ring Laser Gyro or IFOG - same thing cheaper construction)] plus accelerometers. Sets of three of both of these units are mounted at mutual right angles. The combination gives all the directions, rates and accelerations required by all systems on the aircraft - effectively there are no individual units such as a "Normal Accelerometer" as the whole set is used to provide and internally verify the output data.

[Obi Offiah]

- effectively there are no individual units such as a "Normal Accelerometer" as the whole set is used to provide and internally verify the output data.

Thanks ICT_SLB.
I understood this was possible with the modern systems you mentioned above, but not the older gimballed, stabilised platform systems.

I understand there are AHRS units in which the accelerometers are not aligned with the aircrafts axis. A processor converts raw sensor data to the aircrafts pitch roll and yaw axis.

There are AHRS units that only use a set consisting of two accelerometers, (excluding other sets for redundancy) that when processed, output normal and lateral accelerations to the FCC. Although these systems do have the capability to provide longitudinal acceleration data, that capability is not utilised.
Thats why I wonder in general, how common it is for longitudinal acceleration data to actually be fed into FCCs for aircraft control?

Cheers
Obi

[Filler Dent]

Obi, I can tell you for a fact that on a 737 Longitudinal Acceleration is fed into the FCCs and is used for any autopilot mode or stage of flight that alters the aircrafts acceleration in that plane. ie. Take Off, Go Around, IAS/Mach and Vertical Speed.

All modern aircraft use as much data as possible to refine the flight profile for comfort and economy. As has been previously stated, it's all fed along data buses so it really isn't an issue from an installation point of view.

I'm trying to get your question clear in my head though? When you mean "flight control systems" I take it you mean Autopilots? It may be worth finding a copy of E.Pallets Autopilot book if so, that will tell you everything you need to know.

[Obi Offiah]

I'm trying to get your question clear in my head though? When you mean "flight control systems" I take it you mean Autopilots? It may be worth finding a copy of E.Pallets Autopilot book if so, that will tell you everything you need to know.

Sorry Filler Dent, I wasn't particularly clear in my previous posts, I actually meant none autopilot operation.

Obi