Simviation Forum - Something that's been frustrating me...

Reply #30 - Nov 29^th, 2007 at 9:08am

Brett_Henderson Offline
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When things go wrong in IMC (Murphy assures it)... and the tower/ATC are swamped (the norm these days).. the one thing you had better be able to do, is get to a hold and fly it accurately !

More times than not, that hold is a VOR, or a spot referenced by VOR (radial intersection, DME point, etc.)

If you've got that VOR tuned in (and are good at using it); flying to that hold and holding safely is quicker than trying to hold by GPS. That VOR gauge is part of your instinctive scan... flying to a hold by looking at a GPS screen breaks the scan mentality (and that's assuming part of your problem isn't GPS failure related Wink

).

GPS navigation is not a skill, it's not an art, it's not even a task... ANYbody can keep an airplane icon on a moving map line ....

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Reply #31 - Nov 29^th, 2007 at 12:29pm

Nexus Offline
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Brett_Henderson wrote on Nov 29^th, 2007 at 9:08am:

GPS navigation is not a skill, it's not an art, it's not even a task... ANYbody can keep an airplane icon on a moving map line ....

Why should flying be "hard"?

It's ok to pride yourself in knowing how to navigate by VORs, but there's no reason to slander GPS navigation.
Just because it's easy, doesn't make it less OKAY.
Most young pilots I socialize with are pretty handy with the GPS and can program the thing far quicker than looking up various charts for VOR holds.
The nintendo generation of the skies aren't slouches Wink

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Reply #32 - Nov 29^th, 2007 at 1:16pm

Brett_Henderson Offline
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Flying shouldn't be hard.. heck you can hire someone to fly for you...

I'm not slandering a GPS any more than I'd be slandering a calculator.. "why should math be hard ?"

I'm saying that there's no challenge to it.. To quote someone.. "if all you want to do is babysit the aoutpilot while it tracks a GPS course.. that's fine" But if you want to LEARN something... well you know Roll Eyes

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Reply #33 - Nov 29^th, 2007 at 1:20pm

Brett_Henderson Offline
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Quote:

The nintendo generation of the skies aren't slouches

No doubt about that... and a GPS in capable hands is quite a tool.. But even the Nintendo generation need to learn the basics. I wouldn't want to climb onto a regional jet whose pilot wasn't a whiz at VOR navigation..

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Reply #34 - Nov 29^th, 2007 at 1:53pm

Nexus Offline
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Brett_Henderson wrote on Nov 29^th, 2007 at 1:20pm:

Quote:

The nintendo generation of the skies aren't slouches

No doubt about that... and a GPS in capable hands is quite a tool.. But even the Nintendo generation need to learn the basics. I wouldn't want to climb onto a regional jet whose pilot wasn't a whiz at VOR navigation..

And I wouldn't either

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Reply #35 - Nov 30^th, 2007 at 8:44am

Relentless Ex Member

Actually, most Airlines are not completely GPS.

They use to of used an inertial navigation system, atleast, the planes I'm familiar with. Three gyros, one in each axis, with sophisticaed instrumentation to measure the precession of each gyro as it is moved, and to use that data to calculate velocity and heading relative to a starting point (the point of initilization).

INS were later improved with (laser ring) gyros with no moving parts (AHRS - Attitude and Heading Reference System). They, as in the early B757 and B767, the laser ring units were fed updates from, IGS, VOR and TACAN (the DME portion) data for a more accurate self-updating system. Lastly, GPS is now (primarily) used in most jets depending on the airline(r) and whether they've retrofitted their fleet. So, it will make no difference if GPS fails, or if a bunch of Navaids fail. The only use I can see of not navigating by this is if you're flying into an airport that isn't in the nav database or you simply cannot mess with the CDU due to various situations.

You mentioned you use the A340.... well the CLS A340 atleast, uses the default (GPS) planner instead of modelling its AHRS and FMC. That's semi-realistic, but in real life it's not as simple as only GPS.

And lastly, ILS is always used for precision landings. GPS updates are still no where near accurate enough.

http://www.boeing.com/commercial/caft/reference/documents/RNP757767.pdf

EDIT: And any pilot who cannot navigate by VOR is probably retarded.

EDIT2: Corrected faults.

« Last Edit: Nov 30^th, 2007 at 11:29am by N/A »

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Reply #36 - Nov 30^th, 2007 at 9:10am

Nexus Offline
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Actually most airlines use GPS as primary navigation source.
Even the 757 and 767 who has state of the art strap-down triple IRS setups.
They come equipped with three multi mode receivers (MMR's), each includes an ILS and GPS receiver (GPS unavailable on centre MMR).

So inertial nav in a modern airliner provides plenty of data (roll rates, bang anles, slip angles, wind etc), but they are not used as a the primary navigation tool anymore.

The FMC has a negative bias in selecting the INS/IRS as the position source, it would only use it if GPS has failed and there are no usable navaids for scanning.

From the 767 AOM:
"The FMC uses GPS data for position information as long as the GPS is enabled
and the GPS data is valid. If GPS data is not available or is unreliable the FMC
will use Radio or IRS position data"

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Reply #37 - Nov 30^th, 2007 at 9:13am

Relentless Ex Member

No, I'm pretty sure GPS isn't used in most 767s and 757s.

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Reply #38 - Nov 30^th, 2007 at 9:34am

Nexus Offline
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Quote:

No, I'm pretty sure GPS isn't used in most 767s and 757s.

Why would crew training document lie?
The same thing goes for 737NGs, 777's and even the 744.
The aircraft use GPS as the priority updating source of the FMC, not IRS.

GPS was not included in the 757/767 family intitially, but it is a very easy retrofit which many airlines have opted for.

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Reply #39 - Nov 30^th, 2007 at 9:53am

Relentless Ex Member

Most 767s and 757s I've encountered hadn't been retrofitted.

Wait, could you please clarify something for me. w/ FANS-1, is the position updated primarily by GPS, or is it just completely GPS?

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Reply #40 - Nov 30^th, 2007 at 10:34am

Nexus Offline
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The bias is as following when determing FMC position.
Here's the low down.

"· GPS (as installed)
· Two or more DME stations
· One VOR with a collocated DME
· One localizer and collocated DME
· One localizer.

FMC logic selects the GPS position (as installed) as the primary update to the
FMC position. If all GPS data becomes unavailable, the FMC reverts to radio
updating or reference to IRS position.

The FMC position is derived from a mathematical combination of the positions
determined by the IRS, radio, and GPS systems. It represents the FMC’s
estimate of the actual position of the aircraft. Its accuracy depends upon the
accuracy of the position determining systems."

Yeah I know, much mumbo jumbo there, but IRS is usually the last one to be picked in "determine my position" contest

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Reply #41 - Nov 30^th, 2007 at 10:57am

Relentless Ex Member

Ok thanks.

I found out where I got it wrong.... confusing AHRS with INS.... Embarrassed

Are you a pilot in real life?

« Last Edit: Dec 1^st, 2007 at 12:56am by N/A »

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