Resetting?realigning IRS mid flight
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Resetting?realigning IRS mid flight
Hello sirs,
Is it possible to realign the IRS (in this case the B737-300) whilst already in flight? I cant remember the full details now, but I have seen an air crash investigation programme whereby the crew of a 737 did so prior to the aircraft crashing. I am a non flyer myself, apart from the FSX kind and I had a need to reset the IRS myself whilst on a flight and I was unable to do it. Does any real world 737 pilot know the answer??
Regards,
Rory
Is it possible to realign the IRS (in this case the B737-300) whilst already in flight? I cant remember the full details now, but I have seen an air crash investigation programme whereby the crew of a 737 did so prior to the aircraft crashing. I am a non flyer myself, apart from the FSX kind and I had a need to reset the IRS myself whilst on a flight and I was unable to do it. Does any real world 737 pilot know the answer??
Regards,
Rory
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Not possible. You can downgrade it inflight to 'Attitude' only, so all you can get is attitude information with no navigation whatsoever, but once it is turned off, it cannot be re-aligned and is completely unavailable.
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Many thanks Sir. I was actually flying the wilco 737pic, which I understand is quite realistic. I was also, at the time the IRS went down, unable to re engage the autopilot. Interesting then, why the flightcrew in the programme I saw were trying to realign their IRS, but telling me It cant be done would explain why I was unable to do it! Thankyou again.
Rory
Rory
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You have to be stationary to allow it the 10 minutes or so it needs to align. Any towing on the ground, or even rocking in the wind, will prevent any adequate alignment. I recently had to have a 737 towed to face into wind to give it some stability to align at a windy place in Iceland. It then took about 12 minutes. Sometimes it is better to leave it operating than shut it down when the weather is very windy and another crew is taking over. I don't know what program you were watching, but the crew apparently do not understand how it operates!
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I think the episode you are referring to is the Adam Air 574 incident. The a/c needs to be totally stationary while the IRS is being aligned which can take a good 10 to 15 mins. If the NAV mode fails, then the alternative mode is ATT, which requires the a/c to be manually flown. No autopilot as it gets its info from the IRS. And if i'm not wrong, the pilot has to manually enter the a/cs heading every 10 mins or so while in the ATT mode.
The problem in the Adam Air incident is, if i can remember, is that the pilots assumed that the autopilot was flying the a/c while turning the IRS from NAV to ATT causing the a/c to roll out of control.
The problem in the Adam Air incident is, if i can remember, is that the pilots assumed that the autopilot was flying the a/c while turning the IRS from NAV to ATT causing the a/c to roll out of control.
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...is that the pilots assumed that the autopilot was flying the a/c while turning the IRS from NAV to ATT causing the a/c to roll out of control.
Now, I don't fly the 737, however I do fly the L1011 and if, while in cruising flight, the navigation portion of the IRS became unserviceable for some reason, switching from NAV to ATT would not result in the autopilot causing the aircraft to 'roll out of control.'
The autopilot would trip to CWS until the heading mode on the autopilot was selected, then the autopilot selector could once again be moved to the command position, and the autopilot would continue in the heading mode, with manual turns functioning in all cases.
Fail/operational is the term used.
Wonder if the new(er) 737's are similar?
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In the 737, if an IRS fails or is switched to a different mode than NAV, the autopilot trips off. In the 737-300/400 it is technically possible to re-engage the autopilot after setting the IRS transfer switch to BOTH ON L or BOTH ON R, although the IRS FAULT checklist warns crews against such action. In the B737 NG, the A/P cannot be (re-)engaged if only one IRS is operating correctly.
B737s (at least the variants I know) are equipped with two IRSs. The autopilot uses pitch signals from the on-side IRS and roll signals from the opposite IRS, so any IRS failure will affect both autopilots. The cross-coupling provides protection against a dual-axis autopilot upset due to incorrect IRS signals.
The Adam Air accident report is very interesting, but unfortunately rather vague regarding the exact failure mode of the IRS. I think the crew were not sure whether the left or right IRS had failed, and the report is not specific about that issue either (simply referring to "the IRS").
The report suggests that the crew tried to re-align the IRS in flight, but interpreting CVR comments without further evidence is difficult. I wouldn't completely rule out the possibility that they actually tried to re-align the IRU attitude, while operating the system in (degraded) ATT mode. What is apparent is that the crew did not recognise the deviation of the flight path after the autopilot disengagement for a prolonged period of time. When they finally did respond, the aircraft was already in a steep spiralling dive, the excessive bank angle was not corrected (the crew had probably become spatially disoriented), and the airplane became structurally overstressed.
B737s (at least the variants I know) are equipped with two IRSs. The autopilot uses pitch signals from the on-side IRS and roll signals from the opposite IRS, so any IRS failure will affect both autopilots. The cross-coupling provides protection against a dual-axis autopilot upset due to incorrect IRS signals.
The Adam Air accident report is very interesting, but unfortunately rather vague regarding the exact failure mode of the IRS. I think the crew were not sure whether the left or right IRS had failed, and the report is not specific about that issue either (simply referring to "the IRS").
The report suggests that the crew tried to re-align the IRS in flight, but interpreting CVR comments without further evidence is difficult. I wouldn't completely rule out the possibility that they actually tried to re-align the IRU attitude, while operating the system in (degraded) ATT mode. What is apparent is that the crew did not recognise the deviation of the flight path after the autopilot disengagement for a prolonged period of time. When they finally did respond, the aircraft was already in a steep spiralling dive, the excessive bank angle was not corrected (the crew had probably become spatially disoriented), and the airplane became structurally overstressed.
