galaxy flyer

Well, probably aimed at either Adlerdriver or anyone flying airliners. The Collins in the either the Challenger or Global had VGP which armed with the approach loaded, APPR and VNAV selected. It would go VGP with Baro VNAV or LPV path captured, the ALT selector was then ignored by the system and these approaches were flown just like an ILS, with the requirement to add 50’ (DDA) to account for going below the MDA on the miss. There is an OpsSpec deleting the DDA requirement, IF the runway had been 1:34 surveyed.

I’ve been amazed or surprised at the differences between commercial and high-end bizjets avionics. Went into BDL, with the ILS down and flew LPV to land and several airlines weren’t capable.

GF

HIFLYR

We add 50' to the MDA of a V/S CDFA approach and call it a DDA derived descent altitude at FedEx. Some MDAs in VNAV require a DDA also, it is mainly driven by the TERPs assessment of the runway or aircraft specific limitations i.e. non compensated baro for cold weather or remote altimeter settings. We are FOM prohibited from doing a old fashioned dive and drive non-percision approach.
When I attended ARINC 424 meetings for my company and the group was working out the policies for VNAV approaches it amazed me all the differences of avionics and the companies attitudes. Some manufactures foresaw the treating of a MDA as DA in some cases and others said no way and went with PROF to MDA.

galaxy flyer

See OpsSpec C073 for using the MDA as a DA.

OPSPEC/MSPEC/LOA C073, VERTICAL NAVIGATION (VNAV) INSTRUMENT APPROACH PROCEDURES (IAP) USING MINIMUM DESCENT ALTITUDE (MDA) AS A DECISION ALTITUDE (DA)/DECISION HEIGHT (DH).

GF

Adlerdriver

The only mode on any transport category a/c (Boeing, AB, MD) I've flown that ignores the altitude in the ALT selector window is ILS, once the glide slope is captured. I think our VNAV needs to have a "bottom" altitude set in the window to calculate the vertical path accurately. When it stops needing that info exactly isn't something I know off the top of my head. We change it at 1000' AGL, so I know it doesn't need it from that point on down. The biggest thing is that unless we're on an ILS, the altitude in that window is never ignored. So, if MAP altitude was lower than the altitude we intercept the VNAV path on the CDA approach (and we set that), it would be captured on the way down.

We finally went back to selecting MAP alitude upon G/S capture on an ILS last month. Prior to that we were selecting runway elevation (like we would on a CDA non-precision) just to be standardized across all approaches. Then at 1000' AGL, we would select MAP altitude. It was pretty stupid considering probably 99% of our actual approaches are ILS and once I have the G/S captured, the next altitude I want in that window is MAP. Why would anyone come up with a procedure that gives us something to forget later in the approach just so we can do things exactly the same as we do on an approach we hardly actually fly.

galaxy flyer

That’s interesting, because the Collins system in Challengers and Globals use VGP which makes any VNAV approach just like an ILS with respect to the altitude selector. But, yes, in VNAV without VGP, you’d have to set the RWY or the path would end wherever to “bottom” was set in ALT SEL. The old Global had the Honeywell system and no VGP.

GF

EMAW

No the E145 wasn’t capable of coupling to an FMS glideslope, so no VGP.

HIFLYR

The VNAV path is encoded in the aircraft on board navigational database. It starts 50’ above the first brick of the runway and is targeted at 3.0 degrees. If 3.0 will not clear all step downs inside the FAF or final flight segment it will be raised.

Adlerdriver

So, does it care what I have in the FCP window?

HIFLYR

Depends on which avionics mfg. on the A300 it depends on if the appch is activated and in NAV OR LOC* and intercepted the VNAV path, once on the path in PDEC then no it does not look at FCP window as missed apch altitude is there. On the A300 bus with a grey box “honeywell” the minimum entry MDA/DDA just tells the autopilot when to disconnect.

cougar

Some classification of approaches is necessary to address this topic thoroughly. OpSpec C052 now separates approaches into three categories; Non-Precision, APV, and Precision approaches. ICAO PBN is taking it a step further, where approaches are either an xLS, or a Non-xLS. An xLS approach is defined as an ILS, GLS, or MLS. The Non-xLS category is subdivided into either Non-Precision approaches or an Approach with Vertical guidance (APV).

All APV approaches are surveyed using a sloping Obstacle Clearance Surface (OCS) much like an xLS approach. This surface starts at the runway and continues to the FAF. For this reason, all APV approach minima are a Decision Altitude (DA), not an MDA, and the approach chart minima is labeled as a DA. As such, OpSpec C073 does not apply to APV approaches as it's redundant. APV approach types are LPV, RNAV(RNP) or RNP AR, and RNAV(GPS)/RNAV(GNSS)/RNP to LNAV/VNAV minima. Currently, no Boeing aircraft have LPV capability. Not sure if any Airbus aircraft are LPV capable, possibly the A350.

Non-Precision approaches are surveyed with a horizontal Required Obstacle Clearance (ROC) surface, such that no obstacle penetrates the ROC. The MDA must be 250' above the ROC, therefore the MDA is also a horizontal plane and cannot be penetrated by the aircraft unless the runway environment is in sight.

This is where OpSpec C073 applies. It allows operators to set a DA in lieu of an MDA for certain Non-Precision approaches that meet specific requirements. It is detailed, but in summary, if the Non-Precision approach is coincident with a VASI, APV or ILS approach, then the DA in lieu of a MDA may be used if the operator has the OpSpec.

If an operator doesn't have OpSpec C073, or choses to not apply it due to complexity, but still fly a Non-Precision approach using CDFA principles, then a Derived Decision Altitude (DDA) may be used. This is typically 50 feet added to the MDA, to ensure if a missed approach is executed, the MDA horizontal plane is not penetrated.

For Boeing aircraft with VNAV, the MCP altitude must remain below the aircraft to continue in a VNAV PTH descent, until the FMC is in "Approach Logic". Once the FMC is in "Approach Logic", the FCCs ignore the MCP altitude if it is more than 300' from the current aircraft altitude. Earlier FMCs (747-400 FMC) entered approach logic when the first fix of the approach was sequenced. Most modern FMCs on current production aircraft or retrofits like the NGFMC on the 747-400 enter approach logic when the flaps are out of up. Thus on approach using VNAV, the missed approach altitude can be set once the airplane is 300' below the missed approach altitude.

Finally, to help simplify Non-xLS procedures, all Boeing production aircraft are Integrated Approach Navigation (IAN) capable. IAN allows a Non-ILS to be flown exactly like an ILS. For example, for an RNAV(GPS) approach, the approach is selected in the FMC, and when cleared for the approach the crew simply selects the Approach switch. The FMAs will display FAC (final approach course) for the roll mode, and G/P (Glide path) for the pitch mode. Once FAC and G/P are active, the aircraft is coupled, and the missed approach altitude is set. The autopilot maybe left engaged until 100' RA typically. Just like an ILS, the only option to change pitch and roll mode is to select TOGA, or turn off the F/Ds and A/P. The downside is IAN cannot be used for RNAV(RNP) approaches due to RF Legs in the terminal segment, except possibly the 787.