Q A320 FMGS
 

Hello everybody I'm an A320 pilot to be (very soon!) my training hasn't begun yet but it's not far off. So I desided to be a bit ahead of it and started taking a look at the CBT.

While looking at the FMGS preflight imputs, there was something in the PERF page (Take Off) that the CBT doesen`t explain at all (actually the field is always blank throughout the module) and it is "TO SHIFT [ ]"

Well, I hope you guys don´t mind answering this one for me...

Thanks a lot...

Bye.

DownSouth

TO Shift is used when taking off from an intersection to update the FMCG position. The FMCG assumes that when you select FLX or TOGA you are at the threshold of the take off runway and updates its position to that point. If you are departing from an intersection you enter the shift and it updates that you are at the threshold +/- the shift entry.

Apart from the above being true.... my further understanding of it is that... TO Shift is not required to be inserted (or an absolute must to insert) in new A320s having GPS Primary (Nav Accuracy High) since the GPS already knows where it is and thereby updates it's present position.

If GPS is not "Primary", or in non-GPS aircraft, you insert the TO Shift as a figure in Metres "M"... being the distance from the Departure Runway threshold to the Intersection being used to commence the Take-off roll.

Eg.: if the intersection is 400 metres ahead of the threshold, then insert '400' in "TO Shift [ ]".

Maybe there's a bit more to it than what I've mentioned here (and I will check on it)... but we do always insert the Shift, perhaps more to serve as a reminder during the Take-off briefing that we are doing an intersection take-off?

Indeed, with GPS primary, there is no need to enter TO SHIFT.

I will dispute this, but am happy to be corrected. It is a common school of thought and I have never understood it!

On T/O the aircraft uses an FM position that is the mix IRS with a leaning toward a GPS bias. The result being the GPIRS position.

If the GPS fed directly into the IRS, T/O wouldn't be important. But it doesn't.

Without inserting a shift, there will be error in the MIX IRSs position. If it is significant enough, there will be a GPS/POS disagree on T/O- which will be ignored anyway.

Answer fm Airboos on this very topic:
"It seems that in the past the FCOM used to state that takeoff shift entry was not required on the PERF TAKEOFF page if the GPS is available. This statement was applicable to FMS 2: When the GPS PRIMARY is available, the aircraft position update is performed as soon as the takeoff thrust is applied taking into account the GPS position. Due to the accuracy of the GPS position, even, if the takeoff is not performed from the runway threshold, the takeoff shift that may have been entered in the PERF TAKEOFF MCDU page is, as a consequence, not taken into account.
The FCOM statement you refer too is no longer in the FCOM for the following reasons:
- Commonality between GPS and non-GPS equipped aircraft as per SOP;
- To cover the possible non availability of the GPS and therefore, to ensure that the crew will not risk forgetting the TO SHIFT.
Hoping the above clarifies, please feel free to contact this office again, if you have any questions.
Best regards.
Nicolas VITU-BARBIER
A320/A330/A340 Operational Standards"
No need if GPS Primary then.
Regards,
mcdhu

Interesting, I wonder how much other stuff we all put in the MCDU which doesn't do anything!!

Like the T/O trim setting e.g. 2/0.2DN which is only a reminder!!! Airbus, why not link it into the T/O config check????

Excellent, thanks a lot everybody....

Does anybody know if there is any sort of software that simulates an airbus fmgs to practice with??

Well, thanks again! When I start my ground school I'm sure there’ll be many more questions coming up! Just hope not to be too anoying

Happy Landings!

BYE