can someone tell me why my irs keeps saying align after takeoff when ive already done it on the ground

This is copied directly from my old 757 flight handbook....

Alignment & Nav Modes

The alignment mode is initiated by rotating the mode selectors from OFF to NAV. The alignment process takes 10 minutes. If the aircraft is moved during the alignment, the IRS's must be turned off and alignment re-acomplished. The align lights are on until the IRS transfers to the NAV mode.

Anytime during the alignment process, so long as the airplane has not moved, enter the present position. This is normally done through the CDU while preflighting the FMC. With alignment complete and the present position entered, the IRS's transfer automatically to the NAV mode. Once the initial alignment is done and the IRS's are in NAV, a 30 second realignment can be done as long as the airplane is not moved. This is sometimes done at the departure end of the runway just prior to takeoff if the airport chart has the LAT/LONG for that runway position. This is accomplished by selecting ALIGN, entering the exact position in the CDU and then returning the mode selector to NAV.

IRS In-flight Re-start

If an IRS looses both AC & DC power or the mode selector is moved from the NAV position, the alignment to true north is lost & the NAV mode is inoperative. No navigation data can be obtained until re-alignment on the ground. However, the gyro system may be re-started in flight to provide attitude and heading information by selecting ATT.

In case you wanted to know...

The IRS is a big improvement over the INS. The INS had mechanical accelerometers whereas the IRS uses the Doppler shift of light through a ring laser "gyro" to do its sensing. As the plane sits at the gate the IRS's sense the rotation of the earth. From this they can obtain two pieces of information. They know where EAST is from the rotation, so they know where NORTH is! They also sense the rotational speed of the earth and using simple math know the Latitude. The only thing left is to figure Longitude. This is where the entering the exact position comes in. Once you enter the LAT/LONG the IRS knows its position and with its super clock the rest is easy.



FWIW

One of our local high school grads won the Nobel prize in Physics - Dr. William (Bill) Phillips.



I was talking to Dr. Phillips at a party not long ago, and he knew I was a retired airline pilot. He wanted to discuss a new project he is working on. He is experimenting with using atoms instead of lasers in Inertial systems. The use of atoms instead of light photons will increase the accuracy of navigation by many fold. This is because the atoms travel much slower than light and thus the sensing of time difference is more accurate.

You learn something every day.  

Lou

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Mark,

It would be nice to have a choice of 10 minutes or 30 seconds, but I really would rather have the little bugs fixed first.

I myself don't really mind the fast align so I can go flying that much quicker.

Lou

Yup, my brand-x 747-400 is set to fast align too!  

In all the years of flying the planes with INS or IRS, I never really had much of any problems.

One flight however does come to mind. We were the last flight of the day to arrive in KDEN. Our normal gate was occupied by a sick MD-80 so we were directed to the other side of the arrivals wing - about 40 meters across from our normal gate. Since we had minimum rest layover that night, I was surprised to see the plane was at the normal gate the next morning. This proved to be a BIG problem! The ground people hooked up the tug and pulled the plane around to the other side of the terminal without starting the IRS's up. Very bad! You see, these buggers have a real sense of where they are on the planet, and want to be in the same place when next they are started.



The next morning when we got to the plane and started the pre-flight you could smell the trouble. We turned the three IRS's to NAV and entered the gate position. Oh no you don't said the three IRS's, we're over yonder not here. It was too late to move the plane to the other gate area since the passengers were already on board. We spent 30 minutes trying to convince the IRS's that we had moved - NO LUCK! We got the mechanic to come to the cockpit to help.... all he did was scratch his head. We un-powered the whole plane, even removed the ground power.... NO LUCK! After about the 8th time of turning off and trying to align the IRS's I got one to go into align mode, the other two would not cooperate. So I got one to align and put the other two in ATT and that's how we were able to leave town. It was like the good old days in the 727 with two VOR's and DME's!

The one IRS in NAV allowed us to fly a normal flight plan and we had the VOR's for back-up if it went T.U.  

I had heard of all kinds of problems with IRS units that were sent to Honeywell for bench work and then shipped to who knows where to be installed in a plane. It would take all kinds of vodo to get the units to accept their life in a new location.

Lou

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Mark, I really don't know the answer to that question. In school we were told that if you entered the gate position in the IRS panel itself, maybe several times, eventually the unit would accept the new position. I have seen that done with other planes and it the simulator. This 757 was a newer plane with a more up-to-date "Pegasus" system.

When we arrived at our destination, the mechanic came to the cockpit to look at what he could do. He told us that he was going to basically un-install the software and re-install it again.....

                                                               Sorta like FSX!  

So, just one small glitch, caused by ground personnel moving the plane improperly is not too bad for 30 years or so of INS/IRS flying. This system is so much better than anything else we had before.

Lou

Mark,

I'm not so sure that the "ground" people really are aware or well trained enough to know that. 

It would hardly ever come up in their normal day to day operation. In the old days, only mechanics would move aircraft. But mechanics cost more than ramp folks, so airlines switched to rampies to do the tasks that mechanics used to do. For the most part there is no problem, but in this case it really showed how complicated modern planes are and how important good training is to the operation.

Lou

LOU wrote on May 26^th, 2013 at 2:45pm:


Exactly right Lou. I work as a rampie at YUL and besides going to aircraft mechanic school, we received no training regarding aircraft systems. Basically, if a plane has to be moved (other then those for Jazz), a mechanic as to brake ride while we tow.

Just finished an instruments course this semester and we went into great detail on the "generations" and operation of INS. It's a very reliable system, well it must be if has been used for 40 years, right?

Also, as Coolp said, I think IRS was a Honeywell/Boeing term for INS...

Markoz wrote on May 27^th, 2013 at 3:28am:

And they even have a Youtube channel explaining.. things.   http://www.youtube.com/user/irsvideos Just head for the navigation videos. Wait.