6

According to this website the Boeing 737 aircraft use 2 IRS and 2 GPS: IRS L & IRS R. Why? Apparently they are totally independent of each other.

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

Follow-up question: this same website states: "POS INIT is used to enter the aircraft position into the IRS's for alignment". What is meant by "alignment"?

Rodrigo de Azevedo
  • 1
  • 1
  • 9
  • 28
traducerad
  • 779
  • 2
  • 9
  • 16

2 Answers2

10

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

So that if one fails, the other can still be used to complete the flight.

Airbus even have 3.

What is meant by "alignment"?

Initialization of the system and solving the "heading problem" through a magnetometer. But this is a different question.

Federico
  • 32,559
  • 17
  • 136
  • 184
  • Thx for your answer. But if it is only for redundancy and they do not communicate amongs eacht other. How do they compensate the differences in measurements of both systems? 3 isn't that overkill? – traducerad Mar 09 '17 at 09:42
  • @traducerad there is usually a third system that performs what is called "voting and monitoring" – Federico Mar 09 '17 at 09:43
  • "a third system", what system compensates the difference in measurements of both systems? – traducerad Mar 09 '17 at 09:47
  • @traducerad as I said, the "voting and monitoring" one? IIRC, with two INSs, the pilots perform this function, as one uses INS R and the other uses INS L. – Federico Mar 09 '17 at 09:52
  • 5
    @traducerad, The use of 3 identical systems is what is known as triple redundancy. Let's say you have only have 2 systems, and 1 of them begins to show different readings than the other. How do you know which to believe? With 3 systems in total, if 1 begins to show different numbers, you will know that the 2 other ones are more than likely the "correct" ones. – Prodnegel Mar 09 '17 at 14:46
  • 2
    @traducerad - redundancy is what keeps the plane safe should something go wrong with a single system. I'm sure that manufacturers & airlines would love to do away with redundancy because it adds weight, complexity and cost which take away from cargo capacity, range and profit. However, they prefer to not have the bad publicity of planes falling out of the sky due to a failure in a single system. There are probably many single-system failures every day that have essentially zero impact on a flight arriving safely due to the redundant systems. – FreeMan Mar 09 '17 at 15:12
  • Actually, the "heading problem" is solved by waiting long enough to sense the rotation of Earth. Magnetometer is not particularly useful due to the varying declination. – Jan Hudec Aug 02 '17 at 20:47
  • @JanHudec you mean that at (or very close to) the Equator it is not possible to solve it? (Earth's rotation cannot be sensed there). Could be a mix of the two approaches, personally I have seen it done mostly with magnetometers, so I can ascribe to unfamiliarity the reason why I haven't thought of what you mention. – Federico Aug 02 '17 at 20:59
  • 1
    @Federico, no, it is easiest to solve at Equator, because there the axis of rotation lies in the local horizontal plane. The methods are actually both used, because magnetometer gives magnetic heading, and that is used for heading indicator, while the gyroscopic method (gyrocompass is a gyro with attached weight so the axis is horizontal, but I expect IRS to simply spin up all axis gyros, measure the rotation and align the coordinate system accordingly) gives true heading, which is needed by the inertial navigation system. – Jan Hudec Aug 02 '17 at 22:09
  • @JanHudec oh, you're right. I don't know what I was thinking yesterday evening :| – Federico Aug 03 '17 at 04:53
0

Could somebody explain a little bit why they need 2 separate GPS and 2 separate IRS?

Because airliners falling out of the sky or disappearing without a trace is not acceptable from either a political or financial standpoint. Stuff fails and so backups are needed to keep the flight on-track when it does.

Having two systems of each type makes it easier to automatically detect failures, having two different types of system helps avoid systematic issues that might affect two systems of the same type equally.

Follow-up question: this same website states: "POS INIT is used to enter the aircraft position into the IRS's for alignment". What is meant by "alignment"?

Inertial navigation relies on a double integral. The sensors measure acceleration, the acceleration must be integrated numerically to get velocity, and velocity must be integrated numerically to get position. Gravity and rotation complicate the picture even more.

The trouble with integrals and particularly double integrals is they tend to compound errors over time. A constant error in acceleration translates to a linearly growing error in velocity, which translates to a quadratically growing error in position.

So, to maintain acceptable accuracy, intertieral naviation systems must be frequently re-set to the aircraft's real position while the aircraft is staionary on the ground.

Peter Green
  • 1,681
  • 12
  • 15