IRS in ATT

[Pin Head]

Hi

This will give attitude and hdg info only.

What information/data is lost to the a/c compared to when being in NAV mode. Obviously position (lat and long) but anything else?

Thank you.

[NSEU]

This will give attitude and hdg info only.

I've heard it said that V/S will be lost, but I've turned the IRU's to ATT on the ground on a 747-400 and the V/s display looked normal. Has anyone switched all three IRUs to ATT in the air?

You say "hdg info", but heading info is limited to the accuracy of pilot input.

What else does the IRS output (with the assistance of other systems)? Groundspeed (non-GPS derived), wind, drift, Para Visual Display, track... Map displays will be affected (if Track Up oriented), affecting TCAS displays, EGPWS, etc. The Collins Multiscan Radar appears to need full IRU alignment from the tests I've done on the ground.

Of course, results may vary depending on aircraft. Also, displayed data may vary depending on how many IRU's are in ATT mode. e.g. I recall that pilot heading entry wasn't required for, say, a 747-400's Captain's displays, if the L IRU was in ATT mode, because the heading data was automatically sourced from another IRU.

[FE Hoppy]

Typical IRU output these days might include:
Time to Nav
Time In Nav
Pos Latitude
Pos Longitude
Ground Speed
Track Angle True
True Heading
Wind Speed
Wind Direction True
Track Angle Magnetic
Magnetic Heading
Drift Angle
Flight Path Angle
Flight Path Accel
Pitch Angle
Roll Angle
Body Pitch Rate
Body Roll Rate
Body Yaw Rate
Body Long Accel
Body Lat Accel
Body Norm Accel
Platform Heading
Track Angle Rate
Pitch Att Rate
Roll Att Rate
IRU Status
Inertial Altitude
Along Track Accel
Cross Track Accel
Vertical Accel
Inertial Vertical Spd
N-S Velocity
E-W Velocity
Unbiased Normal Accel
Along Heading Accel
Cross Heading Accel

off the top of my head ;-)

[NSEU]

Just looking at the top of my head and these words are written :P ....

In Attitude Mode, the IRS will provide:

• Attitude
• Attitude rates
• Heading
• Acceleration
• Vertical velocity

The last one seems to indicate that V/S will be available.

I'm wondering how Autobrakes/Antiskid will be affected (in the various IRU modes). On many aircraft types, certain functions of Antiskid require groundspeed, but I haven't been able to find out how much the Antiskid/Autobrake system will be affected.

[Chesty Morgan]

Typical IRU output these days might include:
Time to Nav
Time In Nav
Pos Latitude
Pos Longitude
Ground Speed
Track Angle True
True Heading
Wind Speed
Wind Direction True
Track Angle Magnetic
Magnetic Heading
Drift Angle
Flight Path Angle
Flight Path Accel
Pitch Angle
Roll Angle
Body Pitch Rate
Body Roll Rate
Body Yaw Rate
Body Long Accel
Body Lat Accel
Body Norm Accel
Platform Heading
Track Angle Rate
Pitch Att Rate
Roll Att Rate
IRU Status
Inertial Altitude
Along Track Accel
Cross Track Accel
Vertical Accel
Inertial Vertical Spd
N-S Velocity
E-W Velocity
Unbiased Normal Accel
Along Heading Accel
Cross Heading Accel

off the top of my head ;-)
Big head! :}

[FE Hoppy]

Small hat!

[Capt Quentin McHale]

Pin Head,


Searched around and found some old B744 notes and as NSEU said, re- ATT mode the IRS provides....
. Attitude
. Attitude rates
. Heading
. Acceleration
. Vertical velocity


In ATT mode the system is degraded and the IRU's do not provide...
. Position
. Ground speed
. Wind data


Hope this helps


McHale.

[Capt Quentin McHale]

NSEU,


Regarding Autobrakes/Antiskid. On the B744, the IRS and main wheel speed transducers (located in the main wheel axles) send speed signals to the BSCU (Brake system control unit) for antiskid/autobrake control.


The wheel speed signal is the PRIMARY signal and the IRS signal is the secondary. For hydroplane and autobraking protection there is, to some degree, cross referencing of speed signals from both the wheel transducers and the IRS by the BSCU.


McHale.

[NSEU]

The wheel speed signal is the PRIMARY signal and the IRS signal is the secondary. For hydroplane and autobraking protection there is, to some degree, cross referencing of speed signals from both the wheel transducers and the IRS by the BSCU.

This is actually where I was heading. Will hydroplane/touchdown protection be lost as a result of no IRU groundspeed? If so, will the antiskid deem itself unserviceable, and by doing so, inop the Autobrakes (which is dependent on the Antiskid system being serviceable).

My books say

Hydroplane / Touchdown Protection:
Hydroplane / Touchdown protection is provided directly to each of
the aft wheels by comparing wheel speed to airplane ground speed
from the Intertial Reference System (IRS).
• Touchdown protection ensures the rear brakes do not have any
hydraulic pressure applied upon touchdown.
• Hydroplane protection occurs during ground roll to ensure
hydraulic pressure is released at the rear brakes.Protection of the
forward wheels is thus by the locked wheel function.

You can't compare IRU groundspeed with wheelspeed if you don't have groundspeed. There is no mention of primary/secondary references (in relation to this). If there is no touchdown protection, all the tyres may blow on landing (after that, antiskid and autobrake is of not much use). It may be similar to setting the parking brake in flight and then landing (messy).

It may be completely unlike body gear steering system where IRU groundspeed is factored into the logic (yet I know that it will work with with IRUs switched off).

Any comments from PinHead? I'm not even sure why he asked the question ;)

[Capt Quentin McHale]

NSEU,


I see and understand your point, but my notes make reference to the wheel speed transducers as being the primary control input to the BSCU.


Interestingly I found a single sentence explanation for IRS input as follows.... "selected" IRS signals provide ground speed signals for hydroplane/touchdown protection (read into that what you may.)


Nowhere in my notes can I find any explanation if you lose IRS ground speed input. So I can only assume (and we all know where that can lead to) that the BSCU works its magic and only uses wheel speed input, considering that the wheel transducers are signalling the BSCU that they are on the ground and physically spinning up to speed where as the IRU's have zero input on wheel speed.


I also found that wheel speed only, enables antiskid at >8kts and disables at <8kts. Also agree on the body gear steer scenario.


McHale. http://cdn.pprune.org/images/smilies/smile.gif


PS... We've gone a bit off topic, but still learning.

[NSEU]

So I can only assume (and we all know where that can lead to)

Usually the QRH offers some hints at these times (none handy at the moment).

Regarding Antiskid... With no IRU input, if all the wheels on a particular bogey were hyroplaning (i.e. wheel speed zero), would the aircraft see this as the aircraft stopped and keep the brakes applied because the wheelspeed is less than 8kts)?

Regarding Autobrakes, so far I've only looked at the logic for these from one angle: Antiskid problems causing Autobrake problems (My logic diagrams say that if there are any Normal antiskid problems, the Autobrake switch won't arm). Looking at Autobrakes alone....
The IRUs aren't mentioned in the A/B switch (1~MAX) arming logic on the 747-400 (EDIT: Disregard... See correction below), but they are mentioned in the disarming logic. Indeed, during ground tests, the A/B switch won't stay in the latched position with the IRU's off (NB: park brake confirmed off, hydraulics on). I haven't tried this in ATT mode though. One of the inputs to the disable logic is marked "Captain's selected IRS fail". There is no definition of "fail" shown however.

PS... We've gone a bit off topic, but still learning.

Until we know why Pin Head was asking, I think we can indulge ourselves :O

[Capt Quentin McHale]

NSEU,


Regarding Antiskid..... purely hypothetical but I will assume (there I go again) we're talking B744. I think the BSCU will ignore the signals from that particular bogey and pay attention to the other 12 wheel speed transducer signals that say, hey, we are moving at a great rate of knots here, and will continue with normal antiskid ops and the hydroplaning bogey will eventually start to spin up and join the antiskid party.


Regarding Autobrakes.... Agreed.


Autobrakes alone.... "The IRUs aren't mentioned in the A/B switch (1~MAX) arming logic on the 747-400."-- Look again. Agree with ground test scenario.


With regards to Att mode selected, I note that the IRS supplies ground speed, deceleration and pitch angle signals and arming logic requires valid Capts select IRS (ground speed and accel). With the loss of ground speed due ATT selected I wonder if your ground test would pass simply by the test looking at deceleration, pitch angle, air/ground mode, thrust and speed brake lever positions etc?


McHale.

[NSEU]

Autobrakes alone.... "The IRUs aren't mentioned in the A/B switch (1~MAX) arming logic on the 747-400."-- Look again.

oops... missed that one. Maybe I was looking at RTO. I'll amend my message if it's not too late.

I think we need someone with some free sim time to chime in here. i.e. prior to arming autobrakes, put the IRU's to ATT.... look for antiskid messages, then ARM the A/B's. :)