Peter wrote:
For the current UK situation, +V does the whole job which LPV did, and the difference in minima is the difference between poor wx and really poor wx.
Can +V be coupled to A/P to follow some glide path?
Missed approach climb rate excepted but one can deal with that separately e.g. a TB20 can climb at about 10%.
For twins (CAT) referred gradient is OEI climb.
Can +V be coupled to A/P to follow some glide path?
Absolutely!
For twins (CAT) referred gradient is OEI climb.
For sure, for CAT. This actually gets more complicated because twin jets use special “emergency escape” procedures for OEI, designed by a specialised company and AOC approved. Very little is published about this. I am not sure airliners need to meet the SID gradients OEI. They merely need a safe escape route. See APG procedures e.g. here.
Emir wrote:
Can +V be coupled to A/P to follow some glide path?
Depending on the GPS and A/P, yes. Some autopilots use raw ILS glideslope signals from the navigator box and they can’t couple to the +V.
Which autopilots are those, when used with which GPS? Is there a reference? I assume somebody posted something somewhere.
+V is supposed to come out on the same signals from the GPS as LPV does.
Maybe @wigglyamp knows.
Peter wrote:
+V is supposed to come out on the same signals from the GPS as LPV does.
Certainly! I didn’t say anything else.
So any autopilot that could support LPV can support +V, surely.
That’s obviously what I was getting at; if your autopilot is unable to fly vertical guidance then it won’t do +V or LPV or even ILS.
Your KFC 225 is one of the weird ones. It will fly the LPV or ILS or +V but because the GPS presents analog signals as if it is an ILS, the KFC 225 detects a Nav source change from GPS to ILS and reverts from approach mode to a roll mode when the signal source change is detected. The various STC handle this by defining a prompt mode of operation where the GPS indicates to the pilot when to assert the signals for the approach. The operation goes like this. The AP follows the course in Nav mode. When the FAF becomes the active fix (usually when passing the IF), you get the prompt message which tells the pilot to press PROC to get the fake ILS signals asserted, The pilot acknowledges the message and then selects approach mode on the KFC 225. When flown with a GNS 430W, I would ignore the message as I anticipate it on passing the IF, press PROC, then Enter, then select Approach mode on the autopilot. The messaging is similar on the GTN.
Good grief, no wonder I’ve never managed to get the KFC225 to fly a coupled approach. Where does this prompt message appear? I’ve always just pressed the APR button after the penultimate fix. It engages but then invariably finds some reason to disengage later on. Pressing APR after the FAF is too late – I’ve fallen foul of that one too. It just doesn’t engage.
Another method is to feed the KFC225 with analog signals only (it cannot use ARINC429 for VNAV anyway) and use a SN3500 EHSI as a “roll steering converter” which emits the same analog signals whether it is receiving analog ILS data or ARINC429 LPV or +V data. Then the KFC225 thinks it is always flying an ILS (in APR mode) and does not see the ultimate nav source which could be ILS LPV or +V. Not tried this yet, of course 
but this is my understanding and it looks like it will work in that transparent way, and FWIW this is IAW the SN3500 STC.
The point is that +V is a good approach (no pun intended) for the UK. I don’t see any early resolution for LPV here simply because there is no buy-in. Only light GA would use it and it isn’t a revenue source for NATS/CAA/DfT. NATS have stated that not a single penny will be spent on anything for GA unless there is a business case (a rare example: AFPEX was developed because millions were saved on salaries by closing the FBUs) and there is no business case! CAA and NATS are in bed together and the whole lot is owned by the DfT (Dept for Transport). For the sort of airports we have here, mostly no challenging terrain, and with most ATC airports having GPS approaches (the rare exceptions being e.g. Redhill which needs a better runway but the application is being blocked by Biggin Hill on competitive grounds) +V is practically as good as LPV, or indeed ILS.
The most challenging airport I know of is actually my base, EGKA 20, and I was going to go for the 2×IFD540 if they got LPV, but a) they never will and b) +V is as good. The LNAV 800ft DH, applying presumably to +V too, is awfully generous, against the ~550ft of the LPV 20 procedure proposed some years ago which was test flown by a load of people including myself).
I believe an SBAS GPS will provide +V guidance all the way to the tarmac, and a “H+V guided” IAP is always much safer than a “H guided” IAP and with VNAV being flown by setting a GS based VS on the autopilot and monitoring the SDF clearances on the way down.
Gosh what a load of TLAs (three letter acronyms)
I’ve never managed to get the KFC225 to fly a coupled approach. Where does this prompt message appear? I’ve always just pressed the APR button after the penultimate fix. It engages but then invariably finds some reason to disengage later on. Pressing APR after the FAF is too late – I’ve fallen foul of that one too. It just doesn’t engage.
Is this for an ILS or LPV? The ILS is easy: just fly in HDG+ALT mode until near the LOC and then press APR.
johnh wrote:
Good grief, no wonder I’ve never managed to get the KFC225 to fly a coupled approach. Where does this prompt message appear? I’ve always just pressed the APR button after the penultimate fix. It engages but then invariably finds some reason to disengage later on. Pressing APR after the FAF is too late – I’ve fallen foul of that one too. It just doesn’t engage.
The prompt mode operation is a GPS install setting and required if the AP is a KFC 225 by the STC. With the KFC 225, anytime you change the Nav source, the AP will kick out to roll/pitch mode. Normally on an ILS, one be in NAV or HDG mode and selecting APR mode can be done at any time before the approach because the NAV source does not change, so in effect APR just arms the mode so that the GS intercept can occur. With +V or LPV, the source will always appear to change. There may be one additional wire added as well because if one uses ILS energize in the GPS interface, it has to be switched to ILS/GPS Approach.
This is from the GNS 500W Install manual regarding the selection of Prompt mode:
Prompt:
When in GPS mode, the GPS Select discrete is unasserted (open) whenever a GPS approach mode is active and the pilot has enabled the A/P APR Outputs (an associated message is displayed telling the pilot to enable the A/P APR Outputs). This setting will not allow the pilot to select automatic GPS to ILS CDI transitions on the AUX CDI/ALARMS page (only manual transitions are permitted).
For Honeywell (Bendix/King) KFC 225 and KAP 140 autopilots
