Capn Bloggs: I'd love to fly the MD11, and I nearly did, but it slipped through my aeroplane net... but never say never, eh?!
VNAV PATH = basically the vertical profile that is computed, by the FMC (
prior to Top of Descent), as being the most efficient descent path, i.e. when passing over a projected set of waypoints (
as defined within the FMC Legs page), whilst also making allowance for any altitude constraints, and using a range of speeds (
programmable within the FMC and / or 'cost-index' derived).
VNAV SPeedD = the vertical mode that the aircraft reverts to when it decides that it is unable (
for any number of reasons) to follow the idealised VNAV PATH. With suitable pilot input it might be possible to recapture the original VNAV PATH, albeit that this might require more robust inputs, such as 'Speed Intervene', or 'Level Change', or 'Vertical Speed', or even manual flying by way of actually moving the thrust levers & flight controls oneself, forsooth !
Willie Everlearn: In the B757-200's & B767-300's that I flew it was not possible to fly a VOR coupled to an autopilot, i.e. one could couple the autopilot to the 'LOC' (
and indeed at glideslope capture it would automatically engage all 3 autopilots, whether you wanted it to or not) but there was no facility for 'VOR/LOC', i.e. to make the autopilot capture and maintain VOR radial (
unlike the B737) and thus use of the HDG knob was required in order to keep the VOR course bar in the centre.
The B737 Classic & NG does a nice job (
inside the 'glass') of automatically (
i.e. no pilot input required) making localiser sensing 'reverse sense' when one does a back-course localiser approach (
or maybe when doing a circle-to-land from the front course of an ILS approach), though I'd equally contest that one can't couple a back-course localiser to the autopilot in a B737, though I seem to recall that in the B757/B767 one could do that (
i.e. if you pressed the BCRS button on the MCP... it being a button which the B737 does not have?).
Furthermore, in the B757/B767 (
that I flew) there was no GPS fitted and therein one was reliant upon the 3 IRS's and, good as they were, they are not even close when compared to position keeping available via the GPS fitted as standard in a B737NG... and which no doubt accounts for why the B757/B767 suffer somewhat wrt being able to comply with RNP required of most RNAV departures and arrivals ?!
As for exiting out of an approach mode being a 'drama'... all of the aforementioned aircraft types have their idiosyncrasies in this respect.
E.g. For getting out of an ILS APP mode on a B737, my preferred technique is to reach down and de-tune/re-tune the ILS frequency (
it's just a simple button press) wherein, if the autopilot happens to be connected and flying the ILS for you, it will then revert in to Control Wheel Steering (CWS) and maintain which ever amount of pitch & roll it was doing when the tuning button was first pressed (
and it will not re-engage in APP mode, i.e. when you press the tuning button a 2nd time, to re-tune / re-select the ILS. To capture back on to the ILS you'd need to re-select the APP button on the MCP).
One can than select any other mode (
even APP) on the MCP and the autopilot will comply (
as it's still engaged, albeit in CWS until you select another MCP mode), therein this is a natty method of breaking out of an ILS approach, e.g. perhaps into a circling manoeuvre (
especially when one considers how it then subsequently manages the 'glass' / LOC guidance when one approaches the runway from the other end), and doing so with the autopilot always engaged.
To do the same thing on a B757/B767 (
and it's been a few years since I last flew them), I seem to recall that if one was coupled up to an ILS with autopilots engaged, to break-out of the ILS, e.g. into a circle-to-land, all one had to do was press the Go-Around switch, then immediately press ALT HOLD, and then select a different pitch & roll mode on the MCP. We used to use this method when circling to land our B757's at Chambéry Airport (IATA: CMF / ICAO: LFLB) though one needed to remember to press the BCRS button on the MCP (
when on the downwind leg), if one wished for correct LOC sensing as one approached the runway from the other end.
The point here, for either B737/B757/B767, being to use the most accurate method to get aircraft down to circling minima, then breaking off into Circling, all whilst keeping the autopilot engaged to 'reduce workload'.
Wrt to the Cat IIIB capabilities of the B757/B767, you might be interested to know that the B737 can now do that too... indeed we use our HGS/HUD for that very purpose, and therein whilst we are currently only Cat IIIA, we are rapidly heading towards approval for single-engine landings, in 75m RVR,
manually flown!
I totally agree with you about the lack of EICAS in the B737 and, fwiw, the B757 remains my favourite aeroplane !