GPS Implementation (was Re: Peruvian 757 crash -- possible cause reported)

Date:         22 Nov 96 05:48:06 
From:         Ed Hahn <>
Organization: The MITRE Corporation, McLean, Va.
References:   1 2 3
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I hate to take up so much space with all my comments, but here goes. (JAMES H. LUND) writes:

> I've been reading all the usual excuses, but no real reason why the
> good old USA, where GPS was invented, still do not have GPS equipment
> in most commercial aircraft, even for use as a back-up.  Why not
> consider a few reasons for installation?

> Reason 1: GPS also provides reliable information on altitude and speed,
> and makes an extremely useful back up, even when not being used for the
> primary navigation device.  They often also include a memory database
> to warn of infringement of restricted airspace, and emergency features
> to give you quick reference to alternate airports including distance,
> direction, etc.

One thing:  GPS is NOT very accurate in altitude.  With SA on and no
differential corrections, the 2-sigma accuracy is 156 meters, or about
500 feet.  With differential, that goes into the neighborhood of 10
meters (30 feet).  Certainly not better than baro instruments.
Unaugmented, it's not even good enough for following an ATC

The navigation data base is not part of GPS; the FMS already has one.

Also, since an airliner is always going to be on an IFR clearance,
separation from restricted airspace is ATC's job.

Finally, none of these secondary features would be useful in normal
circumstances.  While many of the Nav Data Base features *might* be
useful in an emergency, it's really separate from implementing GPS

> Reason 2: Many of the newer GPS units also have a battery built in,
> which means the system can operate completely independent of all the
> aircraft systems.

No real argument here, except that TOTAL electrical failures,
including the essential (battery) busses, don't happen that often -
anyone aware of such?

> Reason 3:  The price of FAA certified GPS receivers is not that high.
> Even when certified for IFR enroute navigation.

<snip about removing ADF/Loran, and no need for a $20K mil spec unit.>

Actually, you *would* need a high dollar unit because the FAA
certification requirements for equipment going into transport category
aircraft are much more stringent than for GA.  As a "licensed FAA
inspector", I'm sure you are aware of this.

ADF is a requirement to fly NDB approaches (even if no one likes to
fly them). Line pilots still gotta fly an NDB during recurrent
training, and it's still FAA required equipment for Part 121 guys.

> Reason 4: As an FAA licensed pilot, aircraft mechanic and inspector, I
> can assure you that installation costs for GPS are not all that high.
> Especially when they do not need to be integrated into the FMS, EFIS,
> etc.  Total integration of all the instrument systems may be more
> convenient when everything is working; unfortunately people are dying
> in the process of discovering that our near perfect aircraft still
> malfunction occasionally.

OK, one argument:  would you give up your HSI because you wanted the
CDI and DG to be separate in case of failure?  Especially if you had a
completely independent spare HSI, VOR receiver, and DG flying with you?

I don't buy your arguments, but think about it from this angle:  What
additional benefit would the GPS unit provide over and above existing
aircraft systems, especially if none of the other systems can take
advantage of the increased accuracy of GPS?  How would you train the
pilots when to revert to a totally independent system in an emergency?

> Reason 5: Some third world countries have already adopted GPS as a
> standard enroute navigation device.  Several of them have made it
> mandatory.  While most are not certified for precision approaches,
> it is highly reliable and accurate for navigation.

I'm not aware of a single foreign country which has made GPS
*mandatory*; please enlighten us.

> So, why is GPS installation is considered too expensive?  Just how
> many aircraft could be equipped for the price of 1 Boeing 757 aircraft
> lost recently due to malfunctioning cockpit EFIS systems?

<snip about the costs of lawsuits, etc.>

Which aircraft would that be?

1) Dominican Republic - it appears the cause was a blocked pitot
or static port was the contributing factor, and the crew's inability
to function in the abnormal environment the primary cause.

2) Cali, Columbia - all instruments were reported to be functional
when the aircraft hit the mountain.

3) Peru - preliminary reports place a likely factor as a taped-over
static port, which would make all baro instruments (including
standbys) unusable.

Explain which of these accidents would have been avoided with a
GPS on board, and exactly how?  Especially GPS not "integrated into
the FMS, EFIS, etc."

While I certainly am an advocate of GPS technology, there are still
many *REAL* technical and institutional issues to be worked out in its
implementation.  Many of these issues require government action and
are not decisions that the airlines can make on an ad-hoc basis.

It makes *NO* sense for an airline to adopt a limited-capability
system now, when they may need to replace it in a couple of years with
a full-capability system.

Enough for now,

Ed Hahn