IRS Alignment
 

Why the IRS cant be realigned in flight? I know the aircraft need not to be in motion during alignment. But if the IRS loses it’s position, why it stays like that for the reminder of light. Why cant we just provide it with actual position from the GPS so it can realign?

IRS is detecting earth's rotation during alignment, and it would be difficult to do so when doing 500 kts in turbulent air.

There are some military-grade IRUs that are able in-flight alignment though.

Older military aircraft often had INS/IRSs that could be aligned inflight. An example is the USAF C-130Hs with self-contained navigation system (SCNS). Inflight INS alignment requires valid GPS or Doppler velocity sensor data. The alignment process requires that the airplane fly straight and level for a minimum of 1 minute after commanding the alignment, although 5 minutes is recommended. After 5 minutes in alignment, S turns are recommended. The KC-135 fleet was equipped with Carousel IV INSes that also be aligned inflight using similar procedures.

787 IRS may be aligned in flight as well...

this might be dumb question but at ground why the IRS would need to sense the earth rotation when it already have position from the GPS? Or is it just need it for a comparison?

The IRS determines the direction of True North by measuring the direction of apparent wander of its “giros” in 3 directions, due to Earth’s rotation. The rate of apparent wander enables it to determine the Latitude. When we insert present position, it compares the value to its calculated Latitude, but it needs Longitude because that can’t be determined.

That makes sense! So the IRS just compares both latitudes, and if both matches, it will then take/add GPS longitude to it. That explains it, thank you!

Actually, that is incorrect. The GPS provides alignment to grid north, not mag north.

There are many systems, in fact most commercial ac systems, which update realtime the IRU through the GPS, most not more than a 5 minute update.(due to latency)
The kalman filters compensate between the updates. If there is not an update within 15 minutes, most newer IRS system will compensate and keep the headings.... depending on the system...but after their respective time frames, the IRS drift comes into play.
Other factors such as the maneuvers during the update timeframes...if the ac keeps a steady heading and speed, it will compensate longer...make a turn or change altitude, and the accuracy diminishes rapidly...

This is a common misunderstanding. The IRS position is never updated.

The data from all the navigation sources (IRS, GPS, VOR, LOC, DME) goes into the FMC through Kallman filter, where FMC weighs reliability of each data (satellite coverage with GPS, geometry with VOR/DME, etc.) as well as compare outputs of data sources to each other (an IRS which shows completely different position would have lower weight) and calculate what it thinks is the position of the aircraft - so called FMC position, which is then displayed on the ND.

But the IRS itself never gets updated. On the B737, it is not unusual for IRS to be away a mile or two from GPS position after a long flight and Boeing recommends full alignment prior flight whenever possible.

what do you mean by IRS will compensate and keep the headings?

totally agree.. thats what I always thought!

Salute!

At all above! PLZ read up a bit about how inertial systems work and what is needed to "align" them. Several misconceptions and misunderstandings repeated here and incomplete knowledge of how the damned systems work.

First, a few clues to guide you....

- The "system" may be composed of several sensors, not just one magic box.
- The system may not be meant to provide extremely accurate body rates and such for display ( e.g. inertial vertical velocity versus barometric perssure changes) , but basic navigation coordinates with a specified accuracy and update timeframe. You know, where am I?
- an inertial system does not need to be mechanically postioned in "inertial" space to work like they did in the really old days. See "strapdown" systems and RLG's. The tiny one in an AMRAAM is ready to go within a second, and does real well.
- Since the 60's, at least one USAF attack plane could do airborne alignments for navigation and then bombing calculations. A rough initial position was good enuf, then takeoff after 2 minutes or less. The navigation computer then used Doppler info and crude body rates from the inertial platform, which was only good for AHRS at that stage. Kinda like 99% of everything flying. Using doppler and great software, the system could be "aligned" to a "reasonable" state. A radar or visual or flyover position update refined the whole deal and then the thing got better as it then knew lat long, which provided earth rotation rates and better calculation of the wander angle, local level, etc. etc.. I leave it to the reader to research that system and plane.

Gums sends...

I don’t see how an airplane can do a full alignement in the air like some guys are mentioning here. You can have basic mode like attitude and heading but how the IRS will sense the earth rotation during flight???

It gets the information from the GPS.

I if the aircraft maintains a constant heading in level flight for some minimum time and the IRS system is fed ground velocity (Doppler or GPS info) then it can recalculate, from the expected apparent wander, and determine the direction of True North again.

For some mind boggling feats of navigation, see https://web.mit.edu/digitalapollo/Do...progreport.pdf
Apparently one crew managed to input “on launch pad at Cape Canaveral” in mid flight. They realigned the IMU (Inertial measurement unit) using star shots.

As I understand it, Garmin and Aspen GA EFIS systems align anytime they need to. A/C state and position seems not to matter.

AHRS is totally different to INS/IRS.

Attitude Heading Reference Systems provide attitude and heading, strangely enough.

INS / IRS also provides position. Hence the required alignment time.

Actually, and counterintuitively, exactly the opposite is true. The below is from the Challenger 605 FCOM. Note 1 is interesting.

That’s odd. I was so intrigued by what you quoted, I found this reference:
http://www.smartcockpit.com/docs/CL6...ON_SYSTEMS.pdf

”In-Flight Alignment
If a power interruption or a transient system fault occurs in-flight, and a GPS signal is valid, the IRS system will have the capability for re-alignment. During in-flight re-alignment, the IRS ALIGNING message appears on the PFDs.
The FMS will prompt the crew with the SET IRS HDG amber message on the FMS CDU scratchpad (required for FMS navigation – IRS will align without this action). If the crew enters a heading, the IRS will work as a DG until it transitions to the ATT mode, at which point it starts calculating its own HDG.
Typically, the time required for a full alignment in navigation mode is between 10 to 20 minutes. At the beginning of the alignment process, it is recommended that the aircraft maintain a straight and level flight attitude (see Note 1).
NOTE
1. Aircraft maneuvers involving changes in heading increase the alignment time.
2. The alignment time increases as a function of latitude, with the minimum time occurring at the equator, and the maximum time occurring at the poles.

Er, so it can work out your position.
To work out where you are, it does a mathematical calculation, based on your speed over the ground, and heading.
Ignoring ALL other factors , in simples terms your speed over the ground is going to be different if flying east to west or west to east, if flying at a constant air speed.

Ttfn