FD at MDA Non Precision
 

Hey All.

What is the reason to turn off the FD beyond the MDA on a Non-Prec. Approach?

Because the approach is not designed to provide guidance below this point, merely to allow you to acquire the runway and land.

If you are not following the FDs they should be switched off hence removing them.

That´s true but on a Cat 1 approach you also go visual below the DA and there you don´t turn off the FD. So i am wondering why it´s happening with Non-Prec Approaches.

I guess its especially because of the missing vertical guidance?

Depends on the airplane... In the 744, you can get VNAV guidance to the runway. Hence, there is no reason to turn off the flight directors.

Also, for a visual approach I often have V/S -800 set on the MCP as a reference point, once on final. The FD will then indicate deviation from that "normal" rate of descent.

For 99% of the straight-in ILS Category 1 approaches, the localizer and glideslope will lead you precisely to the touchdown zone, so most manufacturers/operators trust it well enough to leave the FDs on until landing. Some airplanes remove them at 50ft (or similar) anyway. The cases when you have troubles following FD on a Cat 1 approach below minimum are rare, but still might happen, like the Singapore B777 incident in Munich (Incident: Singapore B773 at Munich on Nov 3rd 2011, runway excursion).

As far as the non-precision approaches go, you have an enormous amount of variety that goes with them: NDB, VOR, RNAV, offset, straight-in, ... And even a straight-in non-precision approach will mostly bring you from one or the other direction, non of which is completely straight in (unless flown with FMS, assisted by GPS). Not to mention there are multiple lateral and vertical guidance modes one can use when flying NPA: HDG, TRACK, (L)NAV, VOR/LOC (lateral), V/S, FPA, VNAV (vertical). Some of them may be appropriate to use below the minimum (VNAV on some Boeings or FINAL APP on Airbus), but it is much more simple to use the "one for all" rule and just switch off (or recycle on some aircrat) the FDs and fly the rest of the approach visually - and anyway, to continue the approach beyond minimums, you should have sufficient visual references to begin with.

FD below minima are a useful tool by providing pitch guidance/crosschecking during marginal weather or weather/terrain/runway light conditions that may bring visual illusions.

However,over reliance on FD by flightcrews,especially below minima,may lead crews to lose their visual cues(maintaning a constant descent and aiming point) which could lead to unsafe approaches when FD are not providing proper guidance and/or switched off.

I believe in switching off/on FD as a general rule when descending below MDA unless as stated above,during few exceptions.

What Intruder said: i.e.

On some types there are some gotchas with turning the FD's off, and most particularly some real gotchas with "one on, one off"......

Can't speak for other operators/types but in our company not all NPAs are coded all the way down to the runway so following the FD after MDA could lead you astray.

Agreed. Not just the airplane, but the mode(s) too - the FD doesn't have to be coupled to a GPS or FMS to provide proper guidance. NAV and VS does a similar job to NAV and VNAV; one is just a lot more precise for the specific approach.

Its a similar situation with setting of the rad alt. Some companies say set it always to the DH/MDH while others say only for CAT II/CAT III.

It always depends on the gospel being read at the time - AFM/(F)COM/SOP, etc.

Simlply, if your not using them or not using their info, turn them off.