Singlecoil

If your ANP rapidly rises, the IRU’s might have been initialized with a bad position. I don’t know about the other Boeings, but on the 737 it displays a “last position”. Some pilots were using that to initialize the ADIRUs (and some instructors were teaching that). The problem with doing it that way is the last position shown isn’t where you are at the gate, it’s where the airplane transitioned to ground mode on the previous flight. If I had to guess, I would say it is where the jet slowed to about 80 knots or so on the previous landing. You can pull out the 10-9 and use the lat/long tick marks in the margin to see it for yourself. For most airports it isn’t a big deal, but in LAS or ORD you have just told the IRU’s that they are over a mile from where they actually are. Now you lose GPS and suddenly you have an ANP problem. Use GPS position to initialize the IRU’s or if that isn’t working, use the gate coordinates.

Blackhawk

I had one years ago where the variation would be shifted dramatically. Say from E3 to W30. Your location would be accurate over CPs and destination, but not enroute. Because of this, we would not get a GPS caution as it didn't know there was a problem. Took us a few flights to figure out what was going on.

iPilot

I believe the FMS automatically updates the IRS with the runway position when you hit the TOGA button on a Boeing. Only concern with putting in exact coordinates for alignment is if your thrust management computer or auto throttles are inop. Otherwise last position or just the airport coordinates works just fine.

Swedish Blender

Only time my GPS has been jammed was at the corner of Iraq, Iran, and Turkey.

Is that a problem?

pangolin

It’s happening in the US frequently.

JohnBurke

There is absolutely no justification for initializing the IRU without verifying current position. Doing otherwise is poor airmanship.

One should verify what the former residual IRU performance was, if available, before bringing the system up, however, to get an idea of which unit might be the most efficient in the event of a fail down and a need to rely on one without other nav input. Again, just good airmanship to now which unit shows the strongest and most accurate, before starting the flight. I very seldom see anybody do this.

It's a bit like not verifying the fuel upload. I see most look at the fuel indication and if it matches what's wanted, the crew is happy. It seems to be a lost basic that one should look at the previous ending fuel, fuel upload, add them and note any discrepancy that isn't explained by APU burn, etc. Simply things, but important, and should be done every time.

As for jamming issues: while there may come a time when ground based navaids or gone, that time is not today. I've never been an advocate of an all-eggs-in-one-basket approach, and am not today. There are locations, of course, that rely only on RNAV and GPS for approaches, arrivals, departures, etc, but when operating IFR to such locations, good airmanship dictates having options that do use ground based navaids, including alternates in mind. It's been mentioned that these concerns are chiefly for general aviation aircraft. Again, good airmanship dictates having a plan B.

It's possible to paint one's self into a box. Don't do that.

I also fly general aviation, sometimes in aircraft with no nav systems; it's either charts or a handheld GPS (and charts); with loss of GPS, then it's pilotage and dead reckoning.

The latter two still work. If one insists on going IFR, and has only RNAV capability, then we're back to good airmanship, which means having a plan to get to VMC and to fly visually.

Singlecoil

This is a good discussion. Actually, the TOGA button update only works with when the GPS updating is selected off, which is not common. But the important distinction is that it only updates FMC position, not the IRU position. The only time the IRU's receive a position is when they are in align mode. A lot of people think the GPS's are constantly updating the IRU, but that is not the case. It is critical to put your actual position, either from a GPS sensor or from using the gate coordinates, when you are aligning the IRU's. You can hit the POS button on the NAV display and it will show you where the IRU's think they are. Most of the time, the FMC will ignore the IRU position and use GPS position, but if you are in an area where the GPS's are jammed and there aren't many navaids around (think northwest of Vegas) then the FMC will revert to using IRU position. Hopefully it was aligned correctly and not given a position miles away from where the alignment took place.

aviatorhi

Clearly this has never happened to you... while it happens about once every month or two to me.

Rapidly is subjective, but in the span of a few minutes you'll be at a few miles when on inertial only.

rickair7777

It might raise relatively rapidly immediately following GPS loss, but it should quickly settle down at a reasonable (for non-GPS RNAV) value.

If you're oceanic, on INS only, then ANP should very gradually increase over time without GPS. Same for a hypothetical total jamming of all sat and ground based nav over land.

Most of the airplanes we fly used to do RNAV all day long without GPS, and many of them did oceanic with INS only a few decades ago. It shouldn't all just go to poop if you lose GPS.

JohnBurke

Fly better equipment, or learn to properly initialize before you leave the gate.