fix(fms): fix airway entry on alternate flight plan

[BlueberryKing]

Fixes #[issue_no]

Summary of Changes

Fixes a bug where trying to enter airways in the alternate flight plan would cause an exception. The error came from accessing the temporary flight plan if one existed, but not the alternate temporary flight plan.

Screenshots (if necessary)

References

Additional context

Discord username (if different from GitHub):

Testing instructions

Try entering an airway on the alternate flight plan of the active and secondary flight plan.

How to download the PR for QA

Every new commit to this PR will cause new A32NX and A380X artifacts to be created, built, and uploaded.

1. Make sure you are signed in to GitHub
2. Click on the Checks tab on the PR
3. On the left side, find and click on the PR Build tab
4. Click on either flybywire-aircraft-a320-neo, flybywire-aircraft-a380-842 (4K) or flybywire-aircraft-a380-842 (8K) download link at the bottom of the page

[tracernz]

Dev Team QA Report

Discord Username : tracernz
Object of testing : #10506
Aircraft : A32NX
Simulator : 2024
Tier of Testing : 1
Date : 27/01/2026

Testing Process:
Entered NZQN23 ANPOV6B.TIMJO Q787 Y111 WARD6A NZWN34 with NZWN34 UPLUD1R.SABDA Y622 MESIX8B NZCH20 alternate into both active and secondary (starting from scratch on the secondary).

Got stuck on the primary route in SEC:


INSERT* does nothing, and no exceptions.
On returning to SEC and then airways on TIMJO again for a second go, it rejects Q787, again with no exceptions:

Then skipped to SEC ALTN and tried to add the airway there, no dice, same issue with INSERT* doing nothing:

On second try, exiting back to SEC F-PLN then back into airways, and entering Y622 in L1 on the empty page, it brought back some of the previous stuff by the looks:

It even let me enter MESIX again:

Still does not insert.

Testing Results:
Failed

Negatives:

Conclusions:

Media:
Works fine in primary alternate: