Hello,

I have already second flight alert on main panel - NO LAND 3. As a result I cannot automatically land.

Any idea if there is any mistake in my usage or set-up ?

many thanks !

JM

Mistake in setup. Usually caused by letting the power get interrupted between GPU - APU, or APU - Engines.

As Kyle noted above power interruption.

 

Your centre IRS is dead. Re-align.

s

 

As Kyle noted above power interruption.

 

Your centre IRS is dead. Re-align.

 

superb thxs

s

 

 

superb thxs

 

is it possible to re-align during a flight ? (did not manage so far...)

No. You cannot re-align while the aircraft is in motion.

No. You cannot re-align while the aircraft is in motion.

 

Well you can try it, but it's not a good idea, cos it won't work lol. Apparently the pilots of Santa Bárbara Airlines Flight 518 in Venezuela were hoping to fly their ATR-42 straight and level enough to do that, so that their AHRS system would align its gyros. Needless to say it didn't work and they ended up desperately trying to fly through a mountain range in IMC using only their standby compass. Rather unsurprisingly and very sadly for the passengers and crew, everyone on board was killed when they flew into a mountain, which even more sadly, they very nearly managed to pull up over. And all because they didn't wait three minutes for the system to align when the battery came on prior to taking off.

I too got the dreaded IRS C caution yesterday. Good thing I noticed it during pushback, so I was able to realign before starting to taxi. The thing is, I don't recall doing anything differently while switching power that I haven't done in 20 years of flying -400 sims (beginning with PS1). Any clues? Dirty GPU power?

Before removing the GPU (ie. EXTernal power) connect the APU generators [1 and 2]. Your ship will thus suffer no AC power interruption. Your APU of course must be running.

 

PS. As noted above no you cannot re-align, full or quick align the IRS in the air. On the ground and virtually stationary. You 'll have to recycle the IRS mode selector switch to OFF and then to ALIGN->NAV to do so.

s

 

 

superb thxs

 

is it possible to re-align during a flight ? (did not manage so far...)

Is it possible to ask the manual such questions?

I mean, really now! I wondered the exact same thing, because I made the exact same mistake as the original poster of this thread, losing the center IRS alignment. I fired up the FCOM and searched for the answer. Took me no longer than it would waiting for an answer on the internet.

I flew the tutorial flight with the right IRS unaligned (because I accidentally switched it off after alignment) so on final got "Land 2" instead of "Land 3."  Probably would have done an OK autoland but I prefer manual landings so I switched off autopilot.

 

 

 

PS. As noted above no you cannot re-align, full or quick align the IRS in the air. On the ground and virtually stationary.

 

Technical question about IRS realignment:  The 747 (and 777) I believe use VOR/DMEs to cross-check position, as well as GPS data -- why can't this data, which should be very accurate, be used to reset IRS?

 

Thanks,

Mike

 

 

Before removing the GPU (ie. EXTernal power) connect the APU generators [1 and 2].

This is precisely what I do every time. I was using the ground operations function. Maybe there was a power hiccup there. I'll try to replicate.

Mike,

 

The IRS spin-up is to ensure the system is properly calibrated. You can, of course, overwrite the position of the IRS with external data (in reality, I think it's just mixed in with the IRS position and never actually 'overwritten' so to speak), but that won't really address the real issue.

 

Without a proper calibration, you can't be sure of the actual accuracy of the tool. Overwriting the position here and there isn't going to make it suddenly calibrated. It simply makes it accurate for that one, specific, instance, and then it degrades quickly from there.

 

With the IRS, the question (particularly in the case of autoland) isn't so much position, as much as its ability to accurately calculate where you are, based on where you were, plus the inertial vectors from that prior position. In other words, it needs to be able to determine your motion, accurately, using inertial references alone. Without a calibrated start, that inertial reference is not accurate and therefore unreliable.

 

 

The IRS spin-up is to ensure the system is properly calibrated. You can, of course, overwrite the position of the IRS with external data (in reality, I think it's just mixed in with the IRS position and never actually 'overwritten' so to speak), but that won't really address the real issue.

 

Thanks for the explanation! 

 

Mike

 

 

I have already second flight alert on main panel - NO LAND 3. As a result I cannot automatically land.

 

NO LAND 3 advisory does not equate to no autoland. You just don't have the redundancy for a CAT3B approach. But you can still do a CAT3A (LAND2 on both PFDs) and CAT2 (LAND2 on one PFD) automatic approach and landing.

 

The only time you would not be able to autoland would be if you saw the NO AUTOLAND displayed above the PFDs. Autoland also has other certification limitations, but those vary from operators to state requirements. For us, we have a max headwind/x-wind limit of 25kts, and a GS angle limit of 2.5 to 3.25 degrees. Those are for autolands in LVP conditions. Autolands in better than LVP have the same limitations as normal manual landings. 

For the tech gurus....

 

I have the feeling that the IRUs in PMDG are overly sensitive to power changes. 

 

There are very few power transfer combinations on the 744 which result in power breaks. Normally the aircraft will seamlessly transfer power from one source to another. The 747-400 was probably one of the first aircraft to be designed with computer power sources in mind. The designers created a system which transfers power without even a millisecond interruption. Prior to transfer, the new power source is synchronised to the old one.. and for a few moments during the power transfer process, both power sources are put online *at the same time* prior to  tripping off the original power source.

 

The only critical time I can think of is when the IRUs are in the early stages of alignment and the IRUs are testing their ability to transfer between AC electrical sources to backup DC sources. This lasts about 5 seconds only. If the IRUs need backup DC power, they will use the APU "Hot" Battery Bus. To prevent power interruptions during APU start, the IRU DC power is switched over to the Main Hot Battery Bus. The battery switching process involves normal electrical relays and there may be millisecond breaks in power. If this millisecond break happens to correspond to the 5 second IRU AC to DC testing period, only then can I see the IRUs having problems.

 

When running on AC power, the IRUs don't need DC power at all.

 

Cheers

John H Watson