When conducting a full alignment of IRU's when an aircraft is on Battery /Ext Pwr (no engine driven power) - can you start the APU?I know the aircraft has to be 'dead still' during alingment cycle and was wondering if starting the APU creates any vibrations etc that would hinder/interfere with the IRU alignment?Anyone please clarify.

I'm not sure but I don't believe vibrations of that kind would have a lot of impact. Well on many airport I guess the APU would be used probably after alignment, with the aircraft powered by the ground that saves some fuel evn if it is a little. But I don't think it would have any impact, only thing that would have to be checked I guess would be that there is no cut in power to the IRS while in alignment as I'm not sure it could not get troubles. I do believe there is a lot of time where the airplane might vibrate, payload loading, maybe a lot of wind, passengers getting on the plane, so my guess is that the airplane must not be moved but that is probably not to the extent it has to be perfectly still on the ground.

INS's are aligned all the time with APUs either on or off, the start cycle does not affect the align. High winds are a different story though.

INS alignment in reality isn't affected by starting an APU nor by gusts. It is the physical movement of the a/c that is required. Just make sure your parking brake stays on until the ready lights come on. Select NAV and then you're ok to go.Your a/c flight manuel should show you the correct chronological order of attention in your checklists.vololiberista

INS alignment in reality isn't affected by starting an APU nor by gusts. It is the physical movement of the a/c that is required. Just make sure your parking brake stays on until the ready lights come on. Select NAV and then you're ok to go.Your a/c flight manuel should show you the correct chronological order of attention in your checklists.vololiberista

Not true, wind gusts do affect IRS align times, although it is rare, there have been days at work when several crews mention having IRS align issues due to wind at the same airport. And those same crews sometimes requiring that 5 min delay be posted, due to the increased align times.

Not true, wind gusts do affect IRS align times, although it is rare, there have been days at work when several crews mention having IRS align issues due to wind at the same airport. And those same crews sometimes requiring that 5 min delay be posted, due to the increased align times.

Well, if so they would have to be cross wind limiting gusts! They would be rare enough indeed as to not be discussed in the INS manuels.vololiberista

Starting the APU before alignment is just fine. There are many airports that don't have ground power available and you don't want to drain your battery. When no ground power is available, APU's are usually started before programming the FMC CDU. Just make sure parking breaks are on and you'll be just fine.Chad

You will find that the INS gauge is one of the most accurate and realistic for the sim (along with TCAS). It follows its manuel to the letter.You need to check your flight manuel to ascertain the sequence of operations to ensure that NAV mode is selected a.in the correct order and b.BEFORE you move the a/c.(either under power or pushback/tow).Have a look at the manuel for the VC10 below to give you an idea.vololiberista

Well, if so they would have to be cross wind limiting gusts! They would be rare enough indeed as to not be discussed in the INS manuels.vololiberista

It is rather rare indeed, but it does happen. Being a rampie for a few years I was always surprized how much a stationary a/c can shift and float around on itsself due to high winds. -800s Ive seen the tail move around approx a foot or two left to right, if it was a direct x-wind on the tail. Needless to say I wasnt surpized once I moved in to OPS to hear the odd crew reporting align issues in high winds.

Well, if so they would have to be cross wind limiting gusts! They would be rare enough indeed as to not be discussed in the INS manuels.vololiberista

I actully just talked to one of the chief pilots here at work. He said that high winds is somthing to be conscious about when aligning the IRU's. The aircaft "buffets" in the wind which doesnt allow a proper IRS alignment. Its noted in the flight ops manual as well regarding this issue. I dont have one on hand to quote to you. If these conditions are affecting the alignment the flight crew will position the aircraft into the wind to minimize the buffet and alow a proper alignment. Interesting side note. It is "leagal" to leave the IRU's in nav mode up to 25 hours from the time of alignment under certain conditions. The IRU's have to be fully re-aligned before any ETOPS flights. Granted that might not be every airlines SOP to leave them for 25 hours. Greg Keath

My flight manuel states a maximum of 15 hours in continuous use. Obviously since then, that time period has been extended. But it is essential to keep them updated.vololiberista