Post by LOU on May 24th, 2013 at 2:29am
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. 8-)
Lou
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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. 8-)
Lou
Uploaded with ImageShack.us