Is MAP mode for ILS safe?

[Hudson]

During an ILS in the 737 some pilots prefer to have MAP mode on the nav display - others prefer the display to be on HSI mode. Some airlines dictate which set-up to use as above, while others leave it to the pilot to decide.

The primary (ADI) display usually displays the localiser expanded scale if the FD's are on and approach mode selected. The problem occurs if the aircraft is flown outside the limits of the expanded scale. Unless the nav display is on HSI mode, it is not immediately apparent how far in terms of localiser deviation the aircraft has drifted off the ILS track. At low altitudes this could be undesirable.

The MAP mode may give a hint of track error - depending on its scale, but insufficient to alert the pilot of a serious deviation close to, or just outside full scale. Of course, a selection to HSI mode would instantly give the true deviation position. But at low altitudes it probably is not a good thing to be flicking switches to change MAP to HSI or vice verca?

Mind you, both pilots could stay on MAP and simply use the standby ADI ILS for raw data localiser deviation. Trouble with that is the scale is quite small and a significant localiser deviation may go unchecked. Also the position of the standby ADI is not always centre instrument panel.

My preference is for the PF to display HSI mode during an ILS, leaving the PNF to use his discretion on which display information to use.

General thoughts requested please?

[FlightDetent]

No FD/manual flight: I prefer HSI which enables me to assess drift and fly the ILS.

Manual flight with FD: Same applies since I strive to fly the thing and watch FD bars follow the aircraft to keep the skills as opposed to chasing them. If done properly I also find it gives smoother and more precise rides.

A/P engaged: MAP mode for better situational awareness. TCAS blips, WXR returns, position x-check. My aircraft doesn't have DME hold for NAV boxes and there are airport installations (Frankfurt comes to my mind) where you just can't do the magic the usual way.

However we fly to certain airports where FMC POS radio updating is very limited, then it is HSI on one side again. Or both if the MAP is useless.

I do not let go hands off the thrust and wheel below 1000'AGL when PF, I do not touch anything below 500' unles required/requested when PNF. So, the previously selected mode remains.

That being said, I use MAP on 9 out of ten ILS APCHs.

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I don't quite get the part about flying outside expanded LOC scale, when does this happen? What would be the FD commanded deviation at full deflection? I mean, if it is training, OK, but FD off is better training and the above applies. For common OPS, if we're ad hoc not up to the task, why not just stick to the bars/gull?

The book also says:
If the ... LOC deviation exceeds one-half dot expanded scale ..., the respective LOC scale changes color from white to yellow, and the miniature runway stem flashes.

A related question, what do you people select for CATII/IIIA approaches? Do your SOPs cover this? Mine don't.

Looking forward for the comments.