Originally Posted by
hans brinker
Which things wouldn't work if you didn't set MDH?
(overdue reply) If you did not set MDH, the machine would remove the FD's and disconnect AP at MDA + ELEV. Applies to old QFE machines.
https://i.postimg.cc/157RWMT0/IMG-0320.png
It is an oddity by all accounts, but the number of airframes is not negligible. The connection to a part of this thread above is that to discover it needs to be done, your best lead is the "MDH" label in MCDU and in FMA's minima box. Thus it might be prudent to read the "BARO" or "MDA" or "MDH" whilst running the C/L. Just like the QNH / QFE prefix - for some operators.
Not for QFE ops, but for QNH ops with a QFE enabled machine. Apparently, the old ones are actually QFE hardwired with QNH functions on top of the basic circuitry.
The point being made with regards to C/L responses is that it is not defined by the manufacturer and IMHO better staying that way.