737NG go-around waypoint sequencing

[president]

If you are on an ILS approach below 2000 ft (approach mode) and the active waypoint for some reason is a point on the arrival behind you, what happens with the LNAV track once you press TOGA? Will it automatically transition to the missed approach track or will it turn towards the active waypoint behind you. If you could quote the reference to the manual it would be great. Thanks

[172_driver]

It will maintain ground track at the time of pushing TOGA and the roll FMA will be blank. FCOM 4.20.18. The previous sequencing of waypoints doesn't matter.

[Denti]

Depends on customer options. Initially all will behave as pointed out above, those with LNAV auto sequencing for go arounds will automatically switch to LNAV at 400'AGL or immediately if at or above 400' AGL. In the simulator it sometimes leads to immediate turns towards the wrongly sequenced waypoint. Never came close to testing it in the real world though.

[SMOC]

Not sure about the 737 but I assume Boeings are mostly the same for LNAV to capture it must be within 2.5nm of the active leg, so if behind you (likely further than 2.5nm) it "shouldn't" capture LNAV but like all things occasionally odd things happen.

[PPRuNeUser0190]

In the sim I have seen the following scenario happen sometimes:

- crew is in a hold
- then get radar vectors for an ILS (they forget to press exit hold)
- after the go-around and LNAV is pressed (or automatically depending on costumer option) the LNAV guides towards the hold again.

Also, never saw it in real life.

[president]

@172driver. Thanks I'm aware of that. It was the waypoint sequencing I was asking about. Pressing LNAV or not. Would you be able to jump directly to the missed approach routing by pressing TOGA or would the active waypoint remain the old waypoint behind you? As pointed out it may not be possible to select LNAV if the criteria are not met. I flew with a colleague who did not update to the active waypoint. He told me it didn't matter as we would automatically transfer to the missed approach route if toga (and the LNAV) was pressed

[172_driver]

Got ya now! Some people more informed than me about the automatic LNAV sequencing. Not heard about it before so believe our planes don't have it.

[737aviator]

While I've never tried it, I just happened to come across a training document using caution which states that our 737 LNAV will bring you to the active waypoint which could be behind you if you happened to be using a Loc Only in VORLOC.
Albeit we do not have auto LNAV so as part of our missed approach one would call for LNAV or HDG and hopefully would see where LNAV was going to bring you before engaging it. :)

[RAT 5]

Why would you have auto LNAV for G/A?. If the G/A is climb straight ahead = RW track then HDG SEL is required. Also, I seen many missed approach routings programmed incorrectly. Often there are 2 criteria and the turn should be made at the first occurring one. The FMC can not program this and usually it defaults to the DME not the ALT above for the turn. Thus LNAV would not be correct.
I can only vouch for the sim and the active WPT, which maybe behind you, remains so even after TOGA. This is FMC 10.8. No idea about customer options.

[ImbracableCrunk]

I had to get a 737 type/ATPL in Asia for some contract work. The Alteon instructor looked confused when I brought up an active waypoint on an ILS for this very reason. "Never Heard that one before," where his words.

[Denti]

@RAT5, Database database programmed missed approaches have no problems with multiple criteria for turning on a missed approach. We use them for the last 20 years and there are no problems whatsoever with them.

Both Jeppesen and Lido supplied databases do not have any problems there. Can't comment on Navtech as we only used their charts for a short period but still used Jeppesen databases, the quality of the Navtech stuff was horribly bad.

[underfire]

Denti has it with coded MA.
These have been in operation for quite some time, although they would generally be tailored for the aircraft/airline, not in general use unless you pay for them, and they are in the box.