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kjjj11223344

A question for 737ng experts

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I just learned about something in an ng called a "heading reference switch" and its relation to polar navigation. Where is this switch located on the real plane? I have searched on google and can't seem to find it.


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Alfredo Terrero

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I'm no expert, but check the Aft Overhead Panel.


Sincerely,

Chase 

 

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It's contained within the 'IRS' controls in as Chase quoted the aft overhead panel.


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Thanks for the responses guys. Haven't checked this thread as I've had lack of time, but I will check this out. So should I be looking at the IRS area of the aft overhead specifically?


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Alfredo Terrero

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The heading reference switch is up at the back of the overhead if a 737 has it fitted, the switch itself is labelled 'Norm' and 'True'. What it does, is change the reference for the IRS nav system from Magnetic to True North depending on the selection you choose, although in theory it should automatically trip when the aircraft reaches the latitudes where True North should be used as opposed to Magnetic, but apparently Boeing recommend you flip it manually to be on the safe side. However... it is part of a customer option known as the 'Polar Option' (i.e. not all 737s have it, which might be why you can't locate it since it is far less common on the 737 than other Boeing aircraft and more likely to be found on a P8 or a BBJ). To make it feasible, FMC update 10.3 or later has to be on the aircraft's FMC, obviously the heading reference switch must be fitted, and the aircraft must have all of the following stuff working (i.e. there is far less leeway for that MEL malarkey): CDU, left GPS, both IRUs in NAV mode, and both display units working. There are some limits that using the Polar Option engenders, notably you can't use Heading Select or Roll CWS when north of 89 deg 30 min north latitude or when south of 89 deg 30 min south latitude, oh, and it screws up the HGS system too since that uses the other reference mode, so you can't rely on that either when in polar navigation mode. Beyond those latitudes there is an issue aligning the nav systems, since they have to detect the Earth's rotation in order to set themselves up and there isn't enough rotation to detect accurately at those places (so you're screwed unless you manually key the data in, which means you generally need some funky radio gear other than what the average 737 has in order to keep stuff updated properly). Aircraft going over the poles also have to have a lot more safety equipment on board, such as cold weather survival suits, extra radio gear etc, and the crews generally need more training as well, for stuff like cold soaked fuel management and the fact that they might need to tune in some Russkie nav systems or contact operators who use meters for altitude, that sort of thing. Dressing up as RJ MacReady will probably help too. Al


Alan Bradbury

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The heading reference switch is up at the back of the overhead if a 737 has it fitted, the switch itself is labelled 'Norm' and 'True'. What it does, is change the reference for the IRS nav system from Magnetic to True North depending on the selection you choose, although in theory it should automatically trip when the aircraft reaches the latitudes where True North should be used as opposed to Magnetic, but apparently Boeing recommend you flip it manually to be on the safe side. However... it is part of a customer option known as the 'Polar Option' (i.e. not all 737s have it, which might be why you can't locate it since it is far less common on the 737 than other Boeing aircraft and more likely to be found on a P8 or a BBJ). To make it feasible, FMC update 10.3 or later has to be on the aircraft's FMC, obviously the heading reference switch must be fitted, and the aircraft must have all of the following stuff working (i.e. there is far less leeway for that MEL malarkey): CDU, left GPS, both IRUs in NAV mode, and both display units working. There are some limits that using the Polar Option engenders, notably you can't use Heading Select or Roll CWS when north of 89 deg 30 min north latitude or when south of 89 deg 30 min south latitude, oh, and it screws up the HGS system too since that uses the other reference mode, so you can't rely on that either when in polar navigation mode. Beyond those latitudes there is an issue aligning the nav systems, since they have to detect the Earth's rotation in order to set themselves up and there isn't enough rotation to detect accurately at those places (so you're screwed unless you manually key the data in, which means you generally need some funky radio gear other than what the average 737 has in order to keep stuff updated properly). Aircraft going over the poles also have to have a lot more safety equipment on board, such as cold weather survival suits, extra radio gear etc, and the crews generally need more training as well, for stuff like cold soaked fuel management and the fact that they might need to tune in some Russkie nav systems or contact operators who use meters for altitude, that sort of thing. Dressing up as RJ MacReady will probably help too. Al
Thanks Al your posts are always very informative. That explains why I have been unsuccessful in finding this switch on google images.

Boeing777_Banner_Pilot.jpgsig_TheBusIveBeenWaitingFor.jpg

Alfredo Terrero

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