Boeing FMC runway disagree message
 

I would like to know if this messages original purpose was as a result of a safety recommendation

Or purely for the problem of different GPS datums.

As far as I remember in the case of GPS mismatch the message is different, the above case is for the procedure tuned in the fmc against the navaid frequency in the respective box...don't know about any safety recommendation.

get it in 747 pratt/GE in china but not rolls royce powered 747. I fly all three in china

we inhibit GPS for takeoff to stop it

was wondering if it was originally in place for protection against what happened in taipei with singapore air

you need to speak with your navdatabase provider on this.

With RNP procedure designs, I have noted quite a number of errors, especially in China and South America...

If you are in China, with a Boeing/Smiths box, the FMC will not accept an airport elevation over 10,000 feet, so there is a disco there as well...

I'm pretty sure it was intended to help prevent accidents like the SQ one in TPE. The problem is that some countries, notably China, use different (outdated?) GPS datums to the FMC database, causing spurious FMC Runway disagree messages. Consequently, some airlines are in the practice of inhibiting GPS updating during take-off to avoid the spurious messages.

Concur,

The WGS-84 standard is relatively new in many countries, coupled with errors. There is also much 'interpretation' on the location of the end of the runway.

In China, one was always dancing a fine line when doing obstacle and runway survey for procedure design, regarding State 'secrets'.

GPS database errors are not unheard of. Just ask Apple.

We get it every time in a B737-300 going into CDG. Basically when following the STAR passed the end of the runway - message appears. Nav boxes are in AUTO and not tuned to the ILS until on radar vectors.

In the case of China particularly, and maybe South America as well, the warning might be because some of the runways, any runway developments or extensions (or in some cases) whole airports) have not made it into the database yet. You can warnings to Honeywell so that they can update the databases and make them more accurate.

turnandburn I'm sure that this message has been devised for both the rasons that you have mentioned.
It is not unheard of to have a parallel r/w fed in th box and to then mistakenly line up on the other runway and even get take off clearance undetected by ATC or flight crew. Specially in low vis conditions.

another example would be to set up for one r/w at the gate and halfway during taxi the controller offers another closer r/w which will help expedite departure and the crew accept it in a hurry to get airborne asap.

I know all these things sound so silly but providing the fmc with this safeguard is in line with the same logic as adding "landing gear....down" in the landing checklist. It's so obvious but what if there's that one in a million odd that one day someone forgets it. And it has happened.

Regarding different GPS datums, it happens to us regularly in Shanghai which is the only chinese place we fly to. But from reading other posts on this thread, it looks like it's common all over china. Boeing has sent out a bullietin for the 777 to handle such situations.

cheers