Takeoff shift A320

[Speedwinner]

Hi folks,

Nearly all of our A320 have gps and are well equipped. The fctm says that the aircrafts with fms2 inhibit the takeoff shift when they have gps primary. So why do we enter the shift? I don't see any reason for that. Gps failure is pretty unlikely and the time window is very small between line and takeoff power.

Any thoughts?

[Metro man]

With non GPS aircraft, if the shift is not entered it updates the position to the numbers when take off is commenced therefore there will be an error if T/O starts somewhere else. This error is applied as a bias to each waypoint afterwards.

I heard that if a shift is not entered on GPS equipped aircraft, constraints may not be met on the SID as the aircraft has less distance than it thinks to reach a required altitude.

[Speedwinner]

Oh ok never heard about that. So the shift you are entering is important for the climb calculations and restrictions? Is that Right? I was thinking that the shift is only important for the position update at the runway in case you don't have gps. So any mechanics here who can confirm that?

[320wonder]

with "GPS Primary"once Take-Off Thrust is Applied, the FMS automatically updates its position.

[oicur12.again]

"So why do we enter the shift?"

Probably because

"Nearly all of our A320 have gps and are well equipped."

Which means not all have GPS. The procedure is probably in place so that you do the same thing across the fleet and therefore don't forget to do it when flying a non gps machine.

[OPEN DES]

with "GPS Primary"once Take-Off Thrust is Applied, the FMS automatically updates its position.

Incorrect. The opposite is true, without GPS Primary the Take-off update is active.

With GPS Primary there is no requirement to enter the TO SHIFT. However do take into account that if you should encounter a loss of GPS Primary before applying take-off thrust, that you will have an instant map shift. DME/DME updating will take place once airborne, but probably not in time to prevent a late first turn. So no harm in inserting the TO SHIFT anyway.

As mentioned: TO SHIFT does affect the FPLN predictions (i.e. later lift-off), so constraints could be missed as a consequence. Having said that: FMGS predictions are not to be used in the planning stage as primary means of ensuring SID gradient compliance.

[Chris Scott]

Do I detect an assumption by some posters that GPS is 100% reliable and infallible?

[CaptainMongo]

OPEN DES is correct - Thales New FM Pilots Guide Release 1A

"Position update

NO GPS [A/C CONF]

Aircraft with no GPS: An automatic position update at runway threshold is performed when the thrust levers are set in the take-off position. If a take-off shift is entered in the PERF TAKE-OFF page, the FMGC position is updated to the coordinates of the take-off shift.

GPSOPT [A/C CONF]

Aircraft with GPS: No position update occurs when the thrust levers are set to the FLEX or TOGA detent."

[320wonder]

you can look at FCTM/ Supplementary /Navigation Accuracy / Computation of AC Position,


The difference between "Without GPS Primary " and "Gps Primary" is clearly mentioned in there.

[Dan Winterland]

There is no point. it does nothing.

[Carnage Matey!]

Best response I've had on this subject is that it's useful for crew SA when dealing with multiple possible intersections for departure. As far as the aircraft is concerned it's pointless, but it's a good reminder that if your shift doesn't tie up with your intersection then maybe you've got the numbers wrong.

[Metro man]

I wonder if the crew of the QATAR A350 involved in the recent incident entered a t/o shift ?

[Juan Tugoh]

Best response I've had on this subject is that it's useful for crew SA when dealing with multiple possible intersections for departure. As far as the aircraft is concerned it's pointless, but it's a good reminder that if your shift doesn't tie up with your intersection then maybe you've got the numbers wrong.

Totally agree with this interpretation - it enhances situational awareness. Given the number of runway incursions and take offs on taxiways etc, anything that helps you make it a positive and conscious decision to use a particular intersection has to be worth while, if it does nothing technically it is still a valuable exercise.

[BEL1000]

It's not needed to enter T.O SHIFT.

[johndo]

My view on it:
Its an ops procedure if both gps are inop / GPS primary not active, to have a nav accuracy as high as possible.

At takeoff the FM position is updated to the runway threshold position as stored in the database.
A takeoff update requires that the takeoff runway is part of the flight plan. This provides the most accurate position update.
If the takeoff run starts at an intersection, with gps primary not active, enter a takeoff shift on the PERF TO page to refine the takeoff update.
An accurate takeoff update ensures a more precise aircraft position during departure.

[dream747]

Seems like Airbus has just updated the entire FCOM suite. Anyone received it from their company yet?

Apparently now we are to insert TO SHIFT regardless GPS primary or not. FCOM Normal procedures now state so, and under the system description the "IF GPS NOT PRIMARY" note has also been taken away for the TO SHIFT update section.

This is just one of the many changes Airbus introduced in this update :ugh:

[FlightDetent]

FCOM/PRO/NOR/SOP/06/D/TAKEOFF DATA INSERTION (PERF TAKEOFF PAGE)
"TO SHIFT................................................AS RQRD
Enter the takeoff SHIFT distance, if takeoff is to be from an intersection.
This is essential for position updating at takeoff and, consequently, for navigation accuracy."

My oldest FCOM is 2008, with quite few editions between that one and the latest, the sentence is perfectly unchanged, applicable for all A/C in fleet.

Question: Where from did we learn not to enter TO SHIFT w/GPS? It was a long time ago, still I seem to remember that as a change, "from today onwards we will not enter ...."

[compressor stall]

Not released into the wild yet, but our upcoming unadulterated A320 family FCOM release reads "TO SHIFT....AS REQD"

[FlightDetent]

The difference being?

[compressor stall]

Nothing. Same as yours. It's the latest from Airbus confirming that for the previous poster to you. Yours was undated

[dream747]

It is even more clear now with the update that we are to enter the T.O Shift. Under DSC-22_20-20-10 P6/8 they have removed the reference line "if GPS not primary", so that would resolve any previous different interpretations from some guys that we don't have to do so.

Besides that, supplementary procedures now come under the normal procedures tab, FCTM has been reorganised completely, FCOM abnormal procedures section tabs are also reworded completely.

[EGPFlyer]

Our FCOM dated Feb 17 says;

At takeoff, when the FM position is updated to the runway threshold position as stored in the database, possibly corrected by the takeoff shift entered on PERF TO page.
In flight, the FM position approaches the radio position or the GPS position at a rate depending upon the aircraft altitude.
Note:
The FM position update at takeoff is inhibited when GPS PRIMARY is active.

[RUMBEAR]

There was a reference ( that I can't seem to find ) that only required shift distance to be entered when it was greater than 1000'. Anybody have that reference or has it been deleted ?

[FlightDetent]

cs: thanks.

RUMBEAR: to me such practice seems to defy the whole point of POS update at take-off. Never had I seen it written either.