# Why are there two IRS switches on the 737NG?

I've noticed in some videos,that there are two IRS switches on the 737NG. Why is that so and what is the primary purpose of IRS? Also, what is the alignment of the IRS mean?

• There are only two IRS on 737ng? Most aircraft have three of them. Commented Oct 13, 2015 at 7:31
• @JanHudec Yep, "left" and "right". I don't know why left and right rather than one or two. Commented Oct 13, 2015 at 7:40
• It's there to ensure you file your tax return correctly :) Commented Oct 13, 2015 at 11:17
• You know what I mean @vasin1987 Commented Oct 13, 2015 at 16:47

Inertial Reference System is a solid state set of 3 ring gyros which navigate using dead reckoning. The system measures accelerations in and rotation around each of the 3 axis so can calculate where it is by adding how much it has moved to the initial position.

A simple example: The aircraft is stationary pointing North. Over a 10 minute period, it measures the accelerations in the X and Y axis and calculates that is has moved +100 kms in the X axis (i.e. North) and +100 kms in the Y axis (i.e. West). By simple trigonometry, we can calculate that the aircraft is approximately 141.4 kms on a bearing of 45 degrees from it's initial position.

The gyros must be aligned with the aircraft frame of reference when it is stationary. This is the "alignment" process and generally must be completed before the aircraft moves. It may also take inputs of the current position from a manual keyboard or from some other system, e.g. GPS.

More modern IRS fits can align when the aircraft is moving, taking an updated position from GPS.

There are 2 systems on the 737, simply for redundancy.

This slide show gives a good overview.

• @MadhavSudarshan You're welcome. Commented Oct 13, 2015 at 8:22
• Is this alignement a one time event or must this be done regularly? Commented Mar 9, 2017 at 11:27

IRS panel

There are 2 selectors, one for each unit. There are 2 units for redundancy and certification.

(source)

Purpose of the IRS

The IRS (inertial reference system) function is to provide the aircraft location/attitude in flight, computed from the position/attitude sensed during its alignment and rotation/acceleration variations accumulated since the alignment.

An IRS provides a reference to the true north (and often the magnetic north equivalence by reading in a table).

In airliners, inertial references are generally included in a broader computing chain, the ADIRU, which also provides air references calculated from air probes (a.g. airspeed and angle of attack).

How it works

An IRS is composed of 3 gyro-lasers (while INS are based on mechanical gyroscopes) to sense rotation movements, and of 3 accelerometers to detect linear accelerations.

(source)

Alignment

The purpose of the IRS alignment is to detect the starting latitude and attitude, by sensing the Earth acceleration. It must be done when the a/c is at rest. To minimize the sensor error, the values sensed are integrated over a period of time (10 mn or so).

The longitude which cannot be sensed using Earth acceleration is provided through other means (like GPS or last position).

(source)

Attitude only mode

If the IRS is restarted in flight, then only the attitude can be sensed again. The location reference will be lost (this is what means the ATT mode on the selectors).

Use of IRS

IRS are a complement of the GPS, but IRS has the advantage of being independent of any ground or space equipment. This is why there are still found in the avionics of modern aircraft.

Accuracy

Sensing and integration errors accumulates with time after the alignment.

Precision of IRS compared to other systems:

(source)

• I read through the linked source page for the IRS Display image and didn't see anything... Any idea what are the extra segments in the left-hand digits of the numeric displays at the top are for? Commented Oct 13, 2015 at 12:54
• @FreeMan: The left-hand positions must be able to display respectively N/S for latitude and W/E longitude (like this).
– mins
Commented Oct 13, 2015 at 17:22
• Makes perfect sense now, thanks! I was trying to figure out how they were used to make number display more clear, and never considered letters. Commented Oct 13, 2015 at 18:26
• @FreeMan: I realize now that the two groups of digits could be understood as one by IRS, but actually the two groups display a single IRS coordinates (lat, lon). The IRS to display is selected using [SYS DSPL] button. By the way, more than the position can be displayed, the type of information is selected using the [DSPL SEL] button. See slide 52 et seq.
– mins
Commented Oct 13, 2015 at 19:07
• Does the GPS replace the accelerometer or gyroscope during the alignement stage? Commented Mar 9, 2017 at 11:23