737 VNAV Approach Logic
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Join Date: Feb 2013
Location: Wherever LNAV Goes...
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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 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."
Join Date: Apr 2010
Location: Land of beautiful EASA
Posts: 774
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.
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.
Join Date: Aug 2000
Location: Almost horizontal
Posts: 94
In my experience, for ‘standard’ RNP approaches (I.e. not A-RNP or AR), the FMC displayed RNP changes to 0.3 at the FAF. The aircraft could be in on-approach logic prior to this waypoint but the RNP does not change.
given the number of options etc with FMC software it’s entirely possible that others have different experiences???
given the number of options etc with FMC software it’s entirely possible that others have different experiences???
Join Date: Jan 2000
Location: FL410
Posts: 782
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...
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...
Join Date: Feb 2019
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In my experience, for ‘standard’ RNP approaches (I.e. not A-RNP or AR), the FMC displayed RNP changes to 0.3 at the FAF. The aircraft could be in on-approach logic prior to this waypoint but the RNP does not change.
Join Date: Aug 2000
Location: Almost horizontal
Posts: 94
Which is exactly what I was saying and why I specifically stated a ‘standard’ RNAV/RNP approach in my example. The change in RNP relates to the selected procedure and your position relative to it, not on whether the aircraft is in on-approach logic.
so the real question is - how do we definitively know it’s in on-approach logic?
Join Date: Apr 2010
Location: Land of beautiful EASA
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Originally Posted by Too Few Stripes
RNP value changes relative to where you are on the approach, not whether you are in on-approach logic.
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.
Join Date: Aug 2000
Location: Almost horizontal
Posts: 94
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.
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.
Join Date: Jan 2000
Location: FL410
Posts: 782
Originally Posted by Too Few Stripes
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.
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 CAN see when FMC uses an RNP which is only used during approach, such as 0.3 (which is not used enroute...
Join Date: Aug 2000
Location: Almost horizontal
Posts: 94
That was exactly my point.
there is a misconception that the RNP value changing to lower than 1 is an indication of ‘on-approach’. In fact the logic would be in ‘on-approach’ before the RNP displayed changes, we just don’t know when!
i find it odd there’s no indication to the crew as to which logic the FMC is using, it does make quite a difference as to how certain situations would play out.
there is a misconception that the RNP value changing to lower than 1 is an indication of ‘on-approach’. In fact the logic would be in ‘on-approach’ before the RNP displayed changes, we just don’t know when!
i find it odd there’s no indication to the crew as to which logic the FMC is using, it does make quite a difference as to how certain situations would play out.
Thread Starter
Join Date: Feb 2013
Location: Wherever LNAV Goes...
Posts: 40
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!
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!
Join Date: Sep 2005
Location: UK
Posts: 509
Direct from my fcom
”the ANP must be monitored during VNAV NPA. The only indication of the FMC tranisitioning into the “on approach” logic is when the RNP changes to 0.5 on legs page.
If RNP 0.5 is not displayed at this stage the approach should be continued, the “on approach” logic will become active at the latest when descending through 2000ft AAL.”
On another page
”the fmc transitions to on approach when the airplane is within
2nm of first approved waypoint (including approach transitions, arcs, procedure turns)
2000ft of airport elevation whatever occurs first”
”the ANP must be monitored during VNAV NPA. The only indication of the FMC tranisitioning into the “on approach” logic is when the RNP changes to 0.5 on legs page.
If RNP 0.5 is not displayed at this stage the approach should be continued, the “on approach” logic will become active at the latest when descending through 2000ft AAL.”
On another page
”the fmc transitions to on approach when the airplane is within
2nm of first approved waypoint (including approach transitions, arcs, procedure turns)
2000ft of airport elevation whatever occurs first”
