Transition Level in A320

[Boyington]

In the Systems Related Procedures of the FCOMs of A320 it is now mentioned that the crew should NOT modify the default TRANS FL value in the PERF APP Page, if it slightly differs from the Transition Level specified on the approach chart or reported by the ATC.
Can some clarify why we SHOULD NOT modify the TRANS FL value. And what does 'slightly' mean?

If the Transition Altitude of an airfield is 4000ft, and if the default TRANS FL on the PERF page is 4500 and that reported by the ATC is FL55 isn't it better to modify the TRANS FL on the PERF APP page to FL55 so that the pilots will have enough time to reset the QNH and commence the Approach Check List.

[Cough]

Why do you have to wait until the FMGC entered TL to change? Change to QNH/STD when your SOP's dictate, not the aircraft.

Secondly, If you are bumbling along at FL55, you will get occasional notifications to set QNH (when it's not required) as the aircraft ends a few feet below FL55. After years of seeing this notification, people may ignore it when it's slightly more critical.

[dream747]

Do you have a reference for that?

In the normal procedures descent preparation section it states that crew should check or modify the transition level. In certain airfields like HK the transition level changes with different QNH.

Cheers.

[Boyington]

PRO-NOR-SOP-01-50 P27/30 Entering Approach Data

[compressor stall]

PRO-NOR_SOP-16-Descent Preparation.

PERF App page paragraph.

...."Check or modify the transition altitude."

And in SRP - 01 - 50

"check and if necessary modify..... TRANS ALT

Maybe it's your company thing?

[dream747]

Interestingly I don't have the page that you have. Under systems related procedure Entering approach data section, mine does say "Check and if necessary Modify" transition altitude.

[Cough]

I think it depends on what type of FMGC you have (Smiths vs Tales???)

[Metro man]

I heard the new standard will cater for F/L on descent and altitude on climb so you won't be faced with this problem.

[CaptainProp]

PRO-NOR-SRP-01-50 P 25/28

In note below the PERF page illustration:

Note: 1.The default TRANS FL value is computed by the FMS based on: ‐ The Transition Altitude of the destination airport (extracted from the NAV Database), ‐ The QNH value entered on PERF APPR page. Due to the internal FMS computation of altitude constraints, it is rounded to the upper multiple of 5. The crew should not modify this default TRANS FL value, if it slightly differs from the Transition Level specified on the approach chart or reported by ATC.

In some cases perhaps changing the TL will make FMS skipp certain altitude constraints in data base procedures?

For Transition Altitude (PERF page for take off) on page PRO-NOR-SRP-01-10 P 42/50 the FCOM STATES

CHECK/MODIFY the TRANS ALT (transition altitude)

In other words, different procedure for TA and TL.

CP

[Metro man]

Hong Kong airspace bumps the transition FL up by 1000' when QNH goes below 980.