B738 Mach trim

[KristianNorway]

Does anyone know the background for the establishment of Vmo/Mmo with 'Mach trim fail' - light illuminated on the 738?

1. Why isn't the Mmo 0.615M instead of 0.82M?

2. Why is there a Vmo-limit on a mach trim system failure, but no Mmo-limit?

Any information would be very welcome

[Flight Detent]

Hi KN,
Our B737NGs, though limited to M.78 as Mmo, still has:

"Limited to 280KIAS/M.78 when Mach Trim light on, both systems inop."

So, in effect, the limitation is 280 KIAS!

Cheers...FD...

[KristianNorway]

Hello FD,

Thanks for the response. That is what makes me most curious.
As I have undestood the case of Mmo is a question of certification. That Mcrit is in the area of 0.615M, so Boeing had to install the mach trim from that speed even though it maintains longitudinal stability without it.
Hence no restrictions on Mach-number with 'Mach trim fail' - light illuminated on the 738.

But the question of why there is an indicated airspeed - restriction during a Mach trim system failure still stands.