737 VNAV Approach Logic
 

How does the FMC choose the "first approach waypoint" when transitioning to approach logic? I find it is a bit random.

"The FMC transitions to “on approach” when the airplane is within:
• 2 NM of the first approach waypoint (including approach transitions
such as arcs and procedure turns), or
• 2000 feet of airport elevation, whichever occurs first."

The approach is programmed into the FMC navigation as a sequence of waypoints. It is aware where either the approach or transition to an approach (manually selectable from the CDU) begins.

Let's say if you only select the approach in the FMC, the typical waypoints would be CF36-FF36-RW36. In this case, CF36 is the first waypoint of the approach.

If you however select transition to an approach (not arrival), it will look typically like ABCDE-FGHIJ-CF36-FF36-RW36. In this case ABCDE is the first waypoint of the transition/approach, as far as the FMC is concerned.

The easiest way to see whether the FMC is "on approach" mode is by checking RNP - it changes to 0.3 when FMC transitions into "on approach" logic. This assumes arrivals/approaches with standard RNPs.

FlyingStone is correct, as is Too Few Stripes.
Except for the additional vital information both are missing: an approach when coded into FMC database has appended information we cannot see ourselves, but the database creators can.
Some waypoints are labelled as IF or IAF waypoint types, it is these that generate the approach logic, similar to Fly-Over or Fly-By in RNAV codings.

Generally speaking, when crew select an approach from FMS, the first waypoint on approach section of waypoint is coded as IAF, somewhere down the line there is one coded IF.
Some random behaviour is sometimes observed when deleting IAF appended information, without crew realising this as they cannot see it, it can be the cause for odd VNAV output afterwards.
Example: DUB ILS 28 can be selected into FMS without transition, the first waypoint coincides with last waypoint of STAR so overwrites the STAR waypoint (speed/alt) and appends its coding (speed/alt) and IAF logic. When crew selects transition from same waypoint in preflight the FMS draws a procedural hold at same waypoint, the initial overfly is still regarded as IAF. Inflight during descent, crew decide to shorten approach (as in practice no procedural hold is flown reaching the waypoint, and crew are advised to route direct to IAF) so crew connects IF to IAF waypoint again. But now as the waypoint selected (same waypoint but from later in the sequence without IAF appended) is used, FMS does not have an IAF anymore, therefore (when descending in VNAV PTH) is transitions to command FLAPS speed as it cannot determine where IAF is. This causes confusion to crew as aircraft does not show a DECEL point anymore and commanded speed in FMS becomes FLAPS speed.

Knowledge of the appended information can be used wisely when entering the route and/or deleting waypoint...

It changes to whatever the RNP procedure was designed to. I have seen 0.1, 0.3, 0.5.

Easiest check is to see what current RNP value. If <1nm you are in approach logic

It depends on the type of arrival and approach. A conventional arrival based on navaids to an ILS approach is unlikely to have the RNP coded into database, so the FMC would default to standard values and you would be able to see the transition to VNAV approach logic in terms of RNP.

However, in case of RNAV/RNP arrivals, the RNP is most likely coded for each leg already, so the RNP displayed will be what is required for current leg, rather being dependant on FMC to figure it out on its own. With FMC U11 and onwards, you can see the coded RNP for the legs by selecting DATA on the EFIS control panel.

The question of knowing when the FMC is in on-approach logic is still open though?
we’re getting bogged down on what RNP value is displayed. The displayed RNP value is not an indication of being in on-approach logic, it is an indication of the current RNP as programmed in the FMC database and dependent on the approach type and where you currently are on that approach.

You cannot SEE when FMC is in on-approach, afaik.
You CAN see when FMC uses an RNP which is only used during approach, such as 0.3 (which is not used enroute...

Thanks for all the reply's so far.

It's a complete mystery. Last night we switched from VNAV SPD to VNAV PTH at 7.4NM from some random waypoint that I made myself as we were on vectors!

As spoken about before, 2 miles before the IAF it will drop into approach logic, or selection of flap will do it aswell. I know its gone into approach logic when the FMA shows vnav pth, if you hit speed intervent it remains in path. Its more obvious if your in vnav speed at the time and see the change

Be careful with that statement.
VNAV does not remain in PATH because of the approach logic, it stays in PATH when flaps are expected to be selected (when VNAV commands calculated time to flap extension). With flaps selected and using SPD INTV the VNAV logic remains in PATH.

Same as per above

FMA: Thrust Mode, Roll Mode, Pitch Mode

THR REF, LNAV, VNAV SPD:; Pitch controls airspeed

SPD, LNAV, VNAV PATH: Thrust controls airspeed

So I flew for the PNW airline that pioneered RNP for a bit recently, and I was able to meet and befriend one of the gentlemen there who designed the RNP approach system back in the '90s. He and his colleague were responsible for all of the design and proving runs for RNP. He was teaching a class and this question came up. He said, and this is mostly a paraphrase: "there is no way for the pilot to determine when the fmc goes into approach logic (geometric vnav). It is buried in the coding, and for whatever reason, the pilots are not made aware when this change occurs, and it is different for different approaches." Take that for what it's worth, but from what I have learned from people who truly know this system, it isn't for the pilots to know.

Steve or Hal?

yes, it will be different, just like TOD.

https://cimg8.ibsrv.net/gimg/pprune....0b7147510b.jpg

VNAV PATH will compute the distance to the first constrained waypoint. The 'idle' descent is backed off from this point, and will calc TOD. The most common use of this is to set a waypoint at 10K, with a speed restriction of 250, this allows the computer to calc backwards and calc TOD with idle descent to that waypoint. This is by far the most efficient use of the coding. TOD idle to 10K. One can see the perils of having the first coded waypoint as the FAF. (see below)

VNAV geometric is used when there are a number of constrained waypoints, the geometric path is computed at the target speed (not 'idle" descent) This reflects all of the waypoints and speed restrictions. An example of this is waypoint at 10K, spd 250, 5000, speed 180, 3000, speed 160 (or whatever the common ATC ALT/SPD are) The geometric path will be set to maintain these altitude and speed restrictions.

VNAV geometric is used to blend RNP and non-RNP traffic, and to maintain airspace speeds other than idle descent.

Hal. I wish I had recorded that class. He's a very interesting man and great instructor!