Ground Temp in Airbus MCDU INIT Page ?

[321XLR]

So, WHY are we inputting ground temp into INIT Page, Line 6-R?

Doesn't TAT determine its own temperature ?

Some crews input the ATIS temp, others put the OAT on the ramp, others don't put anything in,

I feel the training side could talk a little more about this.

Any feedback is appreciated

[AerocatS2A]

We didn't do it until we got the H3 upgrade. Now we do it because the ground temperature input affects climb performance predictions. I suspect it probably doesn't matter much whether you enter the OAT or ATIS temperature as they'd be close enough not to make much difference.

[321XLR]

Thank YOU Aerocat

[Escape Path]

Originally Posted by AerocatS2A

We didn't do it until we got the H3 upgrade. Now we do it because the ground temperature input affects climb performance predictions. I suspect it probably doesn't matter much whether you enter the OAT or ATIS temperature as they'd be close enough not to make much difference.

Thanks for this. Switched recently to a company with Thales FMS software from a Honeywell one and the information in the FCOM about the use of this was pretty much non existent .

[AerocatS2A]

Originally Posted by Escape Path

Thanks for this. Switched recently to a company with Thales FMS software from a Honeywell one and the information in the FCOM about the use of this was pretty much non existent .

Well, it was actually a Honeywell upgrade (H3) that prompted my company to start entering it. We used to always leave it at 15ēC, then H3 comes along and it's suggested that we enter the ground temp. We also have Thales boxes.

[First.officer]

Have to admit I've been doing this for as long as I can recall (since software mod). I know (as AerocatS2A has already advised) it apparently gives improved climb perf. predictions albeit I can find no FCOM reference other than on FCOM-DSC-22_20-50-10-28-INIT A PAGE that has an entry stating that when the "FROM/TO" fields are utilised (or CO RTE), then the defaulted value is the airport of origin ISA value.

[Escape Path]

Originally Posted by AerocatS2A

Well, it was actually a Honeywell upgrade (H3) that prompted my company to start entering it. We used to always leave it at 15ēC, then H3 comes along and it's suggested that we enter the ground temp. We also have Thales boxes.

Huh... funny. I guess I was flying Honeywell boxes without the update you mention. Still, one of those funny things that are nice to know for us of the most curious kind.

[AerocatS2A]

Originally Posted by Escape Path

Huh... funny. I guess I was flying Honeywell boxes without the update you mention. Still, one of those funny things that are nice to know for us of the most curious kind.

It's a relatively recent update. It features a magenta "staple" on the yoyo in descent and prioritises descent path over speed even when in selected speed. There's some good stuff in it like the ability to swap the secondary and active flight plans so the active becomes the secondary and the secondary becomes the active.

[FlightDetent]

Thanks for the heads-up about H3 making it relevant.

IIRC the GND TEMP prompt appeared on INIT A circa 2017. My current operator has around 40 ships and I am the only one making the entry because of long-time IT affiliation (garbage-in & garbage-out) and a borderline healthy dose of OCD. However, when asked, until today I kept telling dear colleagues that it does not make much of a difference, and they may decide to ignore it to avoid awkward stares from the other captains (and trainers).

Of course, a demonstration of the delta_ISA blending from Ground OAT value to the predicted air-mass value within the first 5000-ish feet of climb (AAL) is duly made afterwards. Hence my persuasion is the entry only fine-tunes the initial climb, which is messed up by the THR RED / ACC / initial SID climb / TMA ATC constraint limits anyway - and as such omitting the value during FMS setup bears causes no harm on any single individual sector.

[STBYRUD]

Boeing skipper here... Apart from the climb predictions, I'd guess that it also provides a reasonable takeoff N1/EPR(?) prediction before accurate TAT is available (that's at least what it does in the 737 and 777), would that make sense?

[Checkboard]

I suspect it probably doesn't matter much whether you enter the OAT or ATIS temperature as they'd be close enough not to make much difference.

While I agree it wouldn't make much difference ... the more accurate one for the airmass calculations would be the ATIS temp. That's the one taken from inside a Stevenson screen/instrument shelter which shields the temperature reading from direct heat radiation from outside sources. Your OAT guage, sitting in the open above that hot tarmac (or covered in blowing snow or whatever) doesn't have that advantage.

I would suspect that's why the box doesn't just scrape the OAT reading, and instead asks you to type it in.

[FlightDetent]

Originally Posted by STBYRUD

Boeing skipper here... Apart from the climb predictions, I'd guess that it also provides a reasonable takeoff N1/EPR(?) prediction before accurate TAT is available (that's at least what it does in the 737 and 777), would that make sense?

I always guessed the N1/ERP is governed by FADEC that has its own sensors, the readout of which is not directly visible on the flight deck. Incidentally, the OAT measurement of apron temperature when parked is quite accurate. Also no issues during taxi.

[STBYRUD]

Once the engines are running definitely, but the Boeing FMC shows predicted takeoff N1 based on your input or OAT from an aspirated TAT probe - so I agree that inputting any value manually should not make any difference for the actual takeoff.

[FlightDetent]

Originally Posted by STBYRUD

so I agree that inputting any value manually should not make any difference for the actual takeoff.

Was that the topic? Rather certain we talked about the at-gate values. The FMGS INIT-A LSK 6R (5?) discussed above is not an input for engine rating on that machine. (full stop)

I should have opened the previous post with: "sounds reasonable. Yet nothing alike is observable on AB where ......"