APDF Reliance on IRS

[QAVION]

Hi, chaps.
After a recent incident where the crew of a B744 took off without their IRS's properly aligned(and had to return to the gate), I was wondering if the autopilot(in any of the basic modes) would be available in IRS ATTitude mode(after an entry of heading on the POS INIT page of the CDU).
On the ground, I wasn't even able to get the FD bars to appear. Any ideas?

Thanks.
Rgds.
QAVION

[Sozjot]

Qavion,
It is nice to see someone posting in here. Been damn quiet. I don't know much about the 744 but I remember Litton IRS/INS could be restarted/semi-alligned in flight in Attitude Reference. I would imagine, from a flight safety angle, that the 744 IRS should be available in ATT REF in the scenario you posed. Not much help, I know.

[QAVION]

Thanks, Sozjot...
Nice to get some kind of response :-) I know for sure that the AH and the compass can be induced to appear in flight..... You would think that that is all the autopilot would need to operate, wouldn't you?(Rhetorical question here).

I've also tried posting on the pilots forum, but so far no luck(Seems odd that they don't know either?).

Anyway, if that fails, I'll try and get some sim time and find out for myself :-)

Cheers.

[spannersatcx]

Following this is what I have pulled from the B747-400 maint manual. Basically from what I understand you can only go to ATT from NAV mode and not straight to ATT mode and that ATT is a degraded mode so will therefore not give FD bars. I could (and probably am) be totally wrong of course.

C.Several systems input data to the IRS. The IRS accepts test initiation data from the central maintenance computers (CMC); altitude, altitude rate and true airspeed data from the air data computers (ADC); and initialization data from the flight management computers (FMC).

D.The IRS outputs data to the following systems:

(1)Flight Management Computer

(2)Autopilot/Flight Director

(3)Integrated Display

(4)Weather Radar

(5)Ground Proximity Warning

(6)Autobrake

(7)Yaw Damper

(8)Airplane Condition Monitoring System Data Management Unit

(9)MAWEA card file (stall warning)

E.Digital data is output on three ARINC 429 high speed busses. The output data is identical on each bus.

F.The IRS provides navigational data as follows to the user systems:

-- Accelerations

-- Angular rate

-- Attitude (pitch, roll, yaw)

-- Ground speed

-- Heading (true, magnetic)

-- Inertial altitude

-- Inertial vertical speed

-- Track (true, magnetic)

-- Position

-- Vertical speed

-- Wind speed, Wind direction, Drift angle


(1)Each IRU provides the guidance reference for the airplane by sensing airplane angular rate and linear acceleration. Position, velocity, attitude, heading, acceleration and body angular rates are computed from the sensor data. A digital computer in each IRU performs all the calculations for IRS. The left IRU is shown on the schematic. The right and center units are similar.


(3)NAV (navigation) - The NAV mode is the normal operating mode for the IRS. In this mode, the IRS performs inertial navigation functions and outputs normal IRS data to be displayed or used by other systems.

(4)ATT (attitude) - The ATT mode is used as a back-up mode when a BITE failure is detected or when both AC and DC temporary power loss occurs in the NAV mode causing loss of alignment. In this mode, only attitude, attitude rates, accelerations and magnetic heading are output to the user systems. Most other words are transmitted as failure warn. Heading will not be valid until a heading value is entered from the POS INIT page on the CDU. The IRUs must be cycled through OFF to re-enter ALIGN or NAV. The implication is that if ATT mode is entered, NAV mode can not be re-entered until after accomplishing a full alignment. There is a two second delay before the IRUs will recognize that ATT mode has been selected. This is to prevent inadvertent selection of ATT mode.
IRS attitude mode

(a)In the attitude mode, the IRU provides a rapid attitude/heading realignment capability. The ATT mode is used if the IRU has lost both its ac and dc backup power for more than 0.010 seconds or if certain types of failures are detected by BITE. During ATT mode, only pitch, roll, rotation rates, accelerations, inertial altitude, vertical speed and magnetic heading (if initialized through the CDU) are valid.

(b)The attitude mode is selected by switching the mode select switch to the ATT position. The IRS ALIGN MODE message will appear on the main EICAS display and the IRU will enter a 30 second alignment period while the reference axis is aligned to the local vertical. The IRU software assumes the airplane is level and not accelerating. If the IRU determines that the airplane is turning, it will stop the alignment and wait for the airplane to stop turning.

(c)Once the alignment is complete, the IRS ALIGN MODE message will be removed and it becomes possible to enter a set heading using the CDU POS INIT page. Once entered, the IRU will output a valid magnetic heading which will track airplane turns. The magnetic heading output will not compensate for the Earth's rotation and will need to be updated periodically. Set heading inputs from the CDU are only accepted by the IRU while it is in ATT mode.