Not sure about tonkas but, on Harrier if the mfcds weren't working then the hud will be off too as it just a mirror of data sent through the mfcds. Normally you will just lose one system such as the tad or tgp page rather than the whole mfcd, if your air data computer was hit and u/s you would lose all flight info but the display and Hudson would still output what it has available, plus a load of cautions and other issues!