JammedStab

Occurrence Summary :

......a Boeing 767-375, registration -----, was enroute from ----------- to Inchon International Airport, Korea. Approximately two hours after departure, two of the Inertial Reference System (IRS) switches were inadvertently moved to the OFF position. The crew declared a PAN PAN due to the degraded navigation/autopilot functions and diverted to PANC Anchorage to conduct a re-alignment of the IRS. The operator determined that the IRS switch detents were worn and could be moved with the application of little resistance or force. The Inertial Reference Mode Panel (IRMP) was replaced by maintenance. A fleetwide inspection is underway and further awareness measures have been put in place by flight operations.


So is there much that can be done in this situation or are you pretty much forced to divert?

Thanks

BOAC

Well, you could recover basic IRS functions with an in-flight re-align, but I think depending on 'ifs and buts' and in those latitudes I would probably divert.

What on earth were two grubby little hands doing up there anyway? The kit is switched on before flight and LEFT ALONE!!

JammedStab

Thanks BOAC,

I should know better but what will be lost and what is recovered after an in-flight re-align. Also what difference does latitude have to do with this?

Thanks

galaxy flyer

If they had Carousel VI IRSs, they could have gone for a full in-flight alignment to NAV! I haven't had the nerve to try it yet, but the book says the GPS position input can align the IRS in flight.

GF

BOAC

An in-flight re-align would give a 'rough' attitude and heading but requires frequent re-entry of mag heading. At high latitudes this could be changing fairly quickly and around Alaska you are much closer to the magnetic 'pole' too.

JammedStab

Thanks,

The QRH for one type of aircraft says for an IRS message......

Place the Mode Selector to ATT(action not reversible). Then maintain straight and level constant heading for 30 seconds. Enter the current heading on the SET IRS HEADING line of the CDU position initialization page. The IRS heading may have to be updated periodically.

I suppose the periodically, depends on the actual amount that your actual heading is changing. That might be dependent on convergance and rate of change of variation.

I wonder if aircraft with GPS onboard still have proper nav capability. If Align or OFF is selected in flight, is that the equivalent of an IRU fault and then only ATT is subsequently available.

BOAC

2 things.

1) I cannot comment on the 'bare bones' report you gave us in post #1 - we do not have enough information on the situation they were in.

2) Re GPS - I know not. I suspect that since GPS is primarily only giving position as far as I know (possibly heading, but not sure) I do not think it will feed into the FMC to enable NAV capability. Providing the POS page can be accessed, however, you will always have a lat and long available, but I am not sure I or most others could navigate too easily with that! Back to a plotting chart and a very sharp pencil I feel.

JammedStab

Thanks BOAC, I am not sure either if having GPS will help or not.

However, There may be a minimum navigation equipment prior to entering oceanic airspace. I seem to remember 1 FMC and 2 IRU but perhaps others know for sure.

FE Hoppy

Depends on the type of IRS fitted.

Old ones - ATT
New ones - align in motion and recover full NAV mode.

I guess they had old ones, and no FE the prevent them playing with stuff they don't understand

Denti

Which is much easier on the 737 as ATT is next to NAV, just pull out and one step clockwise, if the switches are a bit worn it can happen without pulling the switch out and is an easy enough mistake to do on the ground when switching from ALIGN to NAV. Inflight one shouldnt touch those things, except after thorough troubleshooting and when advised by a non-normal procedure.

gums

Salute!

I am having a problem with GPS-aided inflight alignment of the inertial nav function ( not the basic attitude reference). What's the deal?

As long ago as 1968 - 1969 our jet could use the doppler ( no GPS for many moons) for an airborne alignment and then one nav fix from anyplace to have good nav - think less than a mile for course steering and positional accuracy, velocities, ground speed, etc..

@ BOAC and others....... The GPS can provide true ground track and outstanding positional accuracy (how do you think those new smart bombs hit within 5 meters using GPS?). Vertical might be up to a hundred meters off at first, but big deal. Baro data can be filtered to improve the vertical stuff as time marches on. And way up north, lattitude was a biggie until we got good inertials and then GPS.

I am having trouble understanding why over 40 years later we would have problems doing an in-flight alignment if all the nuggets did was turn off the inertial system(s).

Anybody here ever tried an in-flight alignment?

MurphyWasRight

GUMS: Although I do not (at all) know the specific aircraft systems I have dealt with similar "allignenment/startup" cases in other fields.

My guess would be that the initial ground alligenment not only sets current lat/long/height/heading (and others) but may also fine tune
sensor calibration.

This last part is much easier to do when all inputs are static where inputs can be averaged over many samples.

While it woould be possible to save ( at least partially valid) values over a power cycle/reboot it would require extra code and HW as well as create yet another path to verify/certify, and probably was not high on the requirements list.

NSEU

I've tried an on-ground IRS alignment during a maintenance tow on a 747-400 (similar generation). Even on the straight sections with few bumps in the tarmac and minimal speed changes, the IRS MOTION message eventually appears and the countdown restarts (although not always immediately after the selector knobs are set to NAV).

Old and new IRSs? I can't see them retrofitting 767s/744s (with GPS) with new systems that can align in flight simply because of a few cases of operator error. Have any in-flight incidents proved fatal? I seem to recall a PPRuNe message thread on an incident in Nepal.

MurphyWasRight

If it had succeeded I beliveve that when the aircraft stopped at the end of the tow the displayed velocity would have been negative whatever the tow speed was.

galaxy flyer

The ProLine 21 with Carousel V, or VI in Globals, can do an in-flight off the GPS. Book says it might take 20 minutes to complete. Often thought of pulling the #3 IRU C/B (there are no mode selectors, alignment on ground starts automatically on aircraft power up) in flight to see an alignment, but never had the nerve.

GF

gums

Salute!

TNX, Galaxy

I was about to point out the use of the early Carousels in the early 747's. So looks like those babies are keeping up with the technology.

I guess that we military folks were two generations ( not one) ahead in the field, as the jet I described was tested in the late 60's and fielded in 1970. Had both a Doppler and an inertial measurement unit, but the nav functions were done by a central computer ( mod of the Apollo LEM doofer). Only problem with the airborne alignment was getting a nav fix to determine present position, then the "system" took it from there and corrected for several things. Drift was about 1 n.m. per hour, so for a thousand mile trip no big deal, and you could do a position update using radar on a known reference to eliminate the "drift".

@ IGh

First heard about "RNAV" in early 70's, and our system was not FAA certified as such. Nevertheless, we would ask ATC go go "inertial direct" to "xxx" and they would request some TACAN cuts along the way and we would provide them, and tell them what our system was showing for course and distance to "xxx". Next call was "cleared inertial direct to "xxx". Longest route I flew non-stop was from Seattle to South Carolina, and was a piece of cake. We could also read out winds, and the Washington Center would often ask us what we were showing at various altitudes as we climbed, heh heh.

Somehow I have not heard from any of the "heavy" pilots here about their experience with various inertial nav systems, and especially the newer ones with GPS integrated with the inertial sensors.

thanks for putting up with this old dinosaur.

BOAC

gums - what other IRS/Nav systems can or cannot do is interesting but not relevant to post#1, a 767-375 about to set off, I take it, on an ETOPS Pacific route. In my opinion, there would be insufficient NAV info to continue. If you are telling me that the 76-375 will have full GPS capability, including track and altitude and this is fed to the FMC for alignment, then I will accept your assurance. I don't think it has more than POS. Certainly GPS derived track and alt were not available to crews on the (newer) 737NGs I flew or to the FMC (as far as I was aware).

As FE Hoppy says, why/how on earth the knobs got touched I know not.and a good hand slapping from a F/E would have sorted that out.

Capt Claret

I can't comment on the 767 as I've not flown one.

On the Douglas/Boeing 717 there is no inflight realignment possible.

And as to BOAC's question about grubby hands being there, the reinforced cockpit door open/deny knobs on out entire fleet, are located on the adjacent panel to the IRS. The three IRS knobs and the door knob are of similar size and shape, and to open the door, one turns the knob in the same direction as turning the IRS off.

I'm aware of a couple of instances where pilots have gone to open the door but sadly grabbed the wrong knob, and only found out when the autopilot tripped off half the screens went blank, and the door unlock siren failed to sound.

BOAC

That helps explain, CC, but TWO switches?

EMIT

767-300 IRS alignment only on the ground in a stationary aircraft.

Once released the parking brake on an apron that turned out to be not exactly flat: the couple of inches of movement into the chocks was enough to upset the alignment process.

Turning off 2(!!) out of 3 IRS's leaves you without autopilot capability, which would require you to leave RVSM airspace.

Having only one out of three redundant and critical systems available is a good starting point to decide on a diversion to nearest suitable airfield.
(an IRS with reliable attitude is a critical system, after all it is needed for pitch and power flying, n'est ce pas).