Switching between IAS and Mach when in LVL CHG not in sync with FMC

[daemotron]

I noticed one thing where I'm not sure if it's a "works as intended": I mostly use LVL CHG in conjunction with N1 for climbing and descending instead of VNAV. This implies that at one point one has to switch between IAS and Mach. Now what I didn't expect: The B733 does this on its own (on the MCP), but not in sync with indications on the FMS CLB page. When climbing with 284 KT IAS (corresponds to a CI of 40), the automatic cut-over to Mach happens at ~FL260+, resulting in a Mach target of 0.70. At the same time, the CLB page still highlights the IAS speed of 284 KT and switches to Mach no sooner than ~FL295 with a Mach target of 0.73 (as indicated after route and perf are entered and EXECed). On descend, a similar behaviour can be noticed: An automatic switch back to KT IAS happens also too early with ~260 KT instead of some 280ish speed as indicated on the DESC page.

Now on one hand it seems logical that LVL CHG ignores what is set in the FMS (it's not VNAV after all...), but I couldn't find anything about an automatic switching between IAS and Mach...

[Litjan]

Hi,

this is correct, the FL CHG mode will switch at a different point than the FMS. I am not sure what the underlying logic or algorithm is, we simply tried to mimic what the real plane does.

The FMS switchover is a bit more involved - the FMS knows what the desired "new" speed is, and can switch over at the exact right altitude. The FL CHG has no idea what kind of MACH no. the pilot wants to fly after switchover, so it just does it at a "safe" speed. It´s up to the pilot to catch this switchover and adjust if desired.

Jan