I uise thje PMFDG-737.

My questoion ffor tje ,real pilots:

Is the plane aligned onve a day or prior to everyh take off'

THank you Pauil12

It's usually aligned when it starts its day of flying and then it is kept powered up to maintain that. But the system itself periodically crosschecks with various sources (GPS, radio beacon triangulation etc), so it will correct any drift errors to maintain position accuracy when it is in contact with these sources.

4 hours ago, Chock said:

It's usually aligned when it starts its day of flying and then it is kept powered up to maintain that. But the system itself periodically crosschecks with various sources (GPS, radio beacon triangulation etc), so it will correct any drift errors to maintain position accuracy when it is in contact with these sources.

We always re-aligned the INUs on our Gulfstreams during ground stops, because the inertial platform drift error rate increases with time from alignment.  Even though the position is constantly corrected via GPS, you still want to keep the internal drift rate to a minimum.

It depends how long the aircraft is on the ground.

At the airline I fly for, if the flight crew are leaving the aircraft unattended (i.e. finished with that aircraft and no other crew/engineers arrived to take it off them) then the IRS's would be switched off as part of a Secure checklist ran before they left.

If another crew arrived before we'd done it we'd leave it all running for them, if it was engineers we'd ask them and they usually wanted us to leave it as is.

On shorthaul I quite regularly jumped in an aircraft that was unaligned, on longhaul not so much, it was usually teeming with engineers and the aircraft either aligned or aligning.

There is also cost to consider with this too. Leaving an aeroplane powered all night when it is parked up is fairly expensive. And if you want the FMC to keep running and the INS to stay aligned, the aeroplane needs to be powered up. Occasionally it will need to be powered up for other reasons, such as when it is being cleaned or serviced.

So you have to either have it plugged into the FEP (Fixed Electrical Power), or have it connected to a GPU (mobile Ground Power Unit). Depending on the airport's charges, that could cost you about 100 quid per hour for the FEP, or whatever your ground service agent charges for running a GPU on your aeroplane. One of the biggest 737 users - Ryanair - do not use FEPs if they can avoid it, instead they tend to always use GPUs. This is to save on costs and it is also why they don't use airbridges either, instead using mobile steps or the optional built-in airstairs on the front door of their B737 NGs. 

When on the main stands at an airport, the FEP is typically plugged in the moment the aeroplane has shut down its engines and you then signal the crew that they have power connected (the signal is you closing your right fist, then pushing it against your open left hand palm, so it looks like the action of plugging something in). The crew will normally then switch to ground power on the overhead and then shut down the APU.

You can see the FEP in this pic. It is the yellow framed box behind the tow bar. It is attached to a scissor link arrangement which is on wheels so it can be dragged out to the plane or stowed out of the way at the head of the stand. It's difficult to make out, but there is a cable plugged into the underside of the aeroplane:

This is where the B737 is better than the A320 as far as the power socket is concerned. On the 737, there is a little door just below the starboard cockpit window which has three little circular press-button latches, you pop those then you can open the access door for the power socket, and then you can plug the power lead in. This is a good design because the power lead is pretty heavy, and since it goes in horizontally to the socket, it stays there. A little amber light comes on in the socket recess to indicate there is power being supplied. On the A320, the socket is underneath the nose near the nose gear, it has a little push button latch to open the power socket access door.

The bad thing about that, is the plug shoves upwards into the socket and is only held there by the friction of the power lead in the socket's tight fit. Since the power plug and the leads are pretty heavy, what you have to do is make sure that plenty of the power lead is below the socket coiled up, to ensure there is as little power cable as possible hanging straight down since it will be pulling on the plug (there should be a little carabina and support strap on the FEP's plug in order to take the weight, but these are often missing). Needless to say, these plugs can fall out sometimes and if they do when the aeroplane is not running its APU and has ground power selected on the overhead, the plane will lose power, and if that happens when the crew are flight planning, they will not be very happy to say the least, because they'll lose their flight plan and the INS could lose alignment too.

You can see the 737's FEP plug on this pic below when we were working on a KLM 737 during a turnaround, and you can see why this one is unlikely to fall out. The access door can sometimes be a bit annoying to close, but it is a better overall design than the A320, although since the A320 is a lot taller than the B737, the Airbus designers didn't really have a lot of choice other than to put the socket where it is on that their airframe:

Compare this to this A320 where the power lead is going straight up to the underneath of the aeroplane. You can see how thick the twisted dual power cable is, so you can imagine how these could fall out under the weight of them hanging down with nothing other than friction holding the plug in there:

Below is another pic of an FEP still connected to an A320. The guy on the headset is listening for the ATC clearance to push back, when he hears that, he will confirm that the parking brake is set with the crew, then he'll remove the chocks from the nose and he will signal the crew to ensure it is okay to remove the FEP plug. The chances are the APU is running at this point but the jet bridge is still connected, so he will also have to visually confirm that is off before pushing the plane back and also check that it hasn't caused any damage as the last bit of his walkaround check. Note that there is a guy in the tug also monitoring the ATC ground frequency so he too will hear when the aeroplane is cleared to push back, and in any case, the crew will normally put the red anti-collision beacon on the moment they get clearance, so there are a few ways you know when it is good to go.

You can hear the APU when it is running, but this does not mean the crew have remembered to switch the power selector to the APU bus, so you always ask them because if they have forgotten and you pull that FEP lead out, they will lose their flight plan and possibly have to realign the INS as well, and that can take about fifteen minutes (depending on where you are in the world, since it works by measuring the 15 degree per hour rotational drift speed of the planet - there's more movement detected near the equator than there is near the poles, so it aligns quicker in some places than others), so they'd probably lose their departure slot and they would not be happy. This aeroplane in this pic was going to Heathrow from Manchester and the reason I was able to take the pic was because on this occasion I'd been working the high loader to put the cargo can onto it, so someone else went to do the headset and pushback and I was about to head back to the crew room for the next job, or I might possibly have had the marshalling wands and have been waiting to stop the traffic behind the plane when it pushed out, can't remember:

If you look at the big yellow FEP control box on that pic above, you can see the buttons on the thing which start the power up. There are two sets because there are two plugs and two outputs. You can just about make out that the two lights on the right hand side are illuminated, indicating that it is still delivering power.

One thing you will also just about be able to see, as previously mentioned, is that the power lead plugged into the plane is slightly supported by a strap which is intended to take the weight of the cable and stop the lead falling out. Quite often the clip for these things is either broken or completely missing, and even when it is there as on this one, the design of the thing is not exactly brilliant. Airbuses definitely have the worst ground power socket design and it is even worse on the A340 and the A330, because you have to plug two of them in and they too are under the nose like they are on the A320 and A321, so they fall out too sometimes. On the plus side, they are less prone to FOD damage than the 737 NG, since the engines are a bit higher off the ground.

If you go to a major airport at night and see airliners parked up on the main stands, you will observe that most of them have their wingtip nav lights on. This is for safety reasons because trucks will be driving about near them and often this is when engineering work is done on them as well, so you need power for that. But airports don't always want planes parked up on the useful stands taking up space, so quite often what occurs, is a 'tow off'. Typically, the airport will say (for example) they want the plane on Stand 28 to be off there by such and such a time to make way for an arriving aeroplane. So to do that you need to phone an engineer for the airline and tell him or her to come to the stand at X time, they will board the plane and start the APU, you will then signal them to ask  if the FEP can be removed (i.e. make sure they have selected APU power on the overhead). The signal for that is pulling your closed fist away from your open palm to mimic pulling a plug out. They will set the parking brake, you can then connect a tug, remove the chocks and then tell them to release the brakes and you will then tow the aeroplane to a remote stand so the main stand is not taken up by a plane sat there doing nothing all night. This is classed as an aircraft movement, so you also have to phone the control tower to let them know you'll be doing the move, and when you are ready to go, you use the radio with ATC just as you would when taxying an aeroplane around and you let them know when you have arrived, since not all airports have ground radar and not all of them have a control tower where you can see the planes all the time. Normally, the guy on the headset in the tug jumps out of the tug when it is ready to turn onto the stand and then marshalls it to the correct stopping place with some wands. this is because it is hard to see the correct stop point from the tug, and you need the nose wheel on the correct mark so that you know its @rse isn't sticking out off the stand and if there is an airbridge, you know it will align properly with the front door of the aeroplane. You can see the ground markings with the aeroplane types marked on them on the pic of that BA Airbus in the BEA paint scheme.

There has to be someone in the cockpit when towing, so that if the tow bar breaks, the aeroplane can be stopped by the engineer sat in the pilot's seat. You also have to have a second person in the tug connected to the guy in the pilot's seat via headset, so he can monitor things to make sure everything is safe. He will watch the towbar at all times to make sure it hasn't sheared off (towbars have fused shear pins which are designed to break if too much forced is imparted to the nose gear so the plane will not be damaged). There is an unusual radio call for this if it occurs: The guy on the headset will call 'X-Ray, X-Ray, X-Ray'. This is the signal for the engineer to apply the brakes immediately, and also tells the driver of the tug to 'floor it' to get out of the way of the rolling aeroplane, since if the towbar is broken, the plane could roll right over the tug and crush the tug's cab. This happened last year at an airport in Thailand and killed a guy in a tug. This is why you can see a curly yellow lead wire running back to this Airbus on this picture which I took at Manchester during a tow when I was on the headset to the plane. We put chocks on the wire to stop it falling off and tangling on the tug wheels, and it is fed through the tug window to my headset:

Some interesting things about this, are that the tug is connected the other way around when towing from how it it is connected for a pushback. Normally the tug's cab faces the aeroplane when doing a pushback, but on a tow, you reverse it onto the towbar so that when towing, you are driving the tug forwards as this makes it easier to see where you are going on a towing move. One the tugs depicted in most flight sims, they are of the type which has no tow bar, instead they have a little cradle which straddles the nose gear and then lifts that up to enable the tug to steer the plane. Since these types of tug can only connect one way, they have a seat and set of cab controls which can rotate 180 degrees, so all the main seat and steering wheel etc can swivel around a bit like an office chair. The cabs on some traditional tugs can also be hydraulically raised or lowered so the view can be improved when reversing.

If for some reason the aeroplane being towed cannot start its APU or does not have an APU, it has to be escorted by a follow me car when being towed, so there are plenty of flashing lights warning other airport vehicles that you are moving.

When an aeroplane is towed onto a remote stand to get it out of the way overnight, these stands are not well equipped. They have no jetways and not all of them have an FEP. So when you tow a plane to one of these, you also have to take some steps so the engineer can get off the plane. If the plane needs to be worked on, you have to take a mobile GPU as well. These mobile GPUs are diesel-powered and like the FEP, they have a bunch of switches on them to select the required voltage for the aeroplane type (it's a different voltage that is required for turboprops than it is for jet airliners, and it is a different plug too, so there is no confusion).

Wow, that was a long meandering post, but I guess if you like all that realism in your sims and wanted to know about how the power affects the INS, then it might have been worth reading. 🤣

Edited January 16, 20215 yr by Chock

An absolutely epic educational post, Al. Thank you. I wonder if GSX are taking notes?  🍻

On 1/16/2021 at 11:17 AM, Chock said:

if they have forgotten and you pull that FEP lead out, they will lose their flight plan and possibly have to realign the INS as well

That's not been the case on any of the aircraft I've flown on. On the Airbus we regularly had the ground power drop out when we were on a stand (usually during boarding) and there's this almighty clunk! and it all goes dark and quiet. Needless to say, after much swearing (not too loud, an Airbus cabin is really quite quiet without any fans running and the punters don't want to think Gordan Ramsey's flying the plane... ask me how I know) and looking for ground staff to put it back in (or turn it on again) we usually just started the APU. Eventually the APU power would fire up and after much flickering of screens and beeps (of varying urgency... all ignored) the whole thing usually came back exactly as we left it, flight plan loaded, weights, performance and IRS's all aligned. Took a few minutes to go through everything again to confirm it's all correct but the crossword was normally resumed quite quickly.

Another time, on the jumbo, for reasons I won't go into, we had to switch it off (for about a minute) and switch it back on again. We only lost the performance data from the FMC, everything else came back just fine.

I was always lead to believe that the reason we switch to APU (from ground power) before the plug is pulled out it simply to protect the guy doing the pulling. There could be a sizable spark/flash if he pulls it out while there's quite a bit of current going through it. From the aircraft perspective it makes little difference, the Airbus would automatically switch to APU power (if available, obviously), I'm not sure about Boeings but it can't be more than a few button pushes away.

The IRS's have a battery backup for the case of complete electrical failure. They need to stay alive (and aligned) just long enough to get another electrical source up and running (APU or RAT usually). The reason they are switched off if the aircraft is unattended is because if the ground power falls out (or trips or is switched off) then the IRS's will run down this backup battery and render the IRS's u/s. This would slow down the departure of subsequent flights.

Really interesting post Chock, never been taught or even heard of the X-Ray X-Ray X-Ray call and I've had a few tow bar's break. To be fair, both were at JFK where headset phraseology is 'relaxed' at best, even when we nearly ran them over.

13 hours ago, iwebber said:

That's not been the case on any of the aircraft I've flown on...

Yup, that is true, almost all of the time, with it being more of an annoyance than a catastrophe, regardless of the slight delay it absolutely does introduce. but trust me, the plan can completely go; I've seen it occur once - wasn't my fault incidentally since it wasn't on a flight I was on lol - I was walking past it on stand 22 at EGCC on the way for a smoke when it occurred - someone pulled the plug on a SAS 737-600 without confirming it was okay to do so and boom, the flight plan was gone which we learned about in detail when walking back past the stand ten minutes later. I've also heard about it occurring once on a Thomas Cook A330 when the power plug got pulled without clearance from the flight deck to do so. So having seen that occur once and heard about it another time, and this on both Boeings and Airbuses, I always tended to treat it as something which would happen if the power lead dropped out, even though it's more a case that it conceivably could happen, just to err on the side of caution, which is never a bad thing with aviation.

Some airlines are pickier than others with headsetting as you say. I noticed a few trends in that: BA crews were generally professional and chatty but not 'stuffy' about things. Air France crews were often pushy and would frequently not adhere to instructions, doing things like releasing the brakes before you said it was okay to so and such. Most Vueling crews barely even pretended to follow procedures, Turkish ones as well, SAS crews were usually pretty friendly, KLM crews as well, and so on. You pretty soon got used to which ones you had to be strict with and which ones were more professional, but it was as well to be aware that your manner and adherence to procedures and safety was going on record on that CVR.

Edited January 18, 20215 yr by Chock