Hi all,

 

I'm having a blast with my newly acquired 737NGX, even more so with FS2crew installed...

 

I have an issue, though : this happens every time, even with no external weather, FS2crew not started, just the aircraft :

 

- Take off from Rouen airport (LFOP) rw04, using SID CEN 3N which says (official chart) : climb MAG 042°. At 1000 (488) turn right MAG 271° to intercept and follow RDL 241° ROU (MAG 241°) to ELBOX then RDL 083° CEN (MAG 263°) to CEN.

- FMC fully configured with take off RW04, SID activated, LEGS checked showing proper waypoints as per the SID description

- LNAV/VNAV armed on the ground, then CMD A selected at 400ft above ground, which translates to 920ft altimetric

- Planes starts to follow the LNAV path perfectly, executing a right hand turn at 1000ft to reach 271° magnetic heading

 

Then things go wrong right before reaching the next waypoint, which in the FMC is a calculated (INTC) waypoint, for the ROU RDL 241° intercept. Every time I am about 2nm from the intercept point (LNAV still active of course, correct path visible in the CDU), plane starts to turn strongly right to a heading of 330° approximately, goes beyond the waypoint and then starts turning back left, overshooting the 241° radial before re-aligning with the correct path. What could be wrong ?

 

I have tried putting a speed restriction down to 180kts at (INTC) waypoint, thinking it could be a case of flying too fast to intercept the radial properly, but it does change anything...

 

If I take off from the other direction (RW22) with the reciprocal SID (CEN 3S), everything goes fine and the planes captures the 241° radial perfectly and smoothly...

 

Has anybody got any idea what could cause this, and how to fix it ?

 

Thanks for your feedback...

 

T. Simon

First off you should not be engaging the AP that low most real pilots fly it up to 10000 feet. Second is the first waypoint active in your FMS? Are you heading toward the active waypoint?

 

 

First off you should not be engaging the AP that low most real pilots fly it up to 10000 feet. Second is the first waypoint active in your FMS? Are you heading toward the active waypoint?

 

The aircraft can be handed over to the AP from 1500ft. In a few cases it is actually recommended to let the automagic handle the aircraft on departures/arrivals due to the high traffic density at certain airports, it's simply more precise. 

Has anybody got any idea what could cause this, and how to fix it ?

I just tried the SID on runway 4, and had the exact same issue. Turned to about heading 330 and then turned back on course. It appeared that the Intercept waypoint remained active the entire time during the turn. It must not be a problem specific to your plane or procedure, because it is happening on other computers. It could be a navdata issue, bug, or just basic pilot error.

One workaround to this problem would be using heading select and VNAV in combination until you are established on the radial. You could also attempt using VOR LOC to intercept the radial once you are on the 271 heading to intercept.

As for the engagement for the autopilot, it is normal to engage at 400AGL in the NGX as per the FCTM. The minnimum altitude for autopilot engamenent in the -600/-900ER is 400 feet AGL. Sure, many pilots choose to hand fly depending on the procedures being flown, but the case or RNAV departures, LNAV and VNAV are sometimes required immediately after takeoff. Autopilot engagement at a low altitude also helps to reduce workload during during the departure profile. (As also said by the above post)

 

 

low most real pilots fly it up to 10000 feet.

 

I'm not saying your wrong, but respectfully that comment is not necessarily correct (at least in the United States). While pilots of airliners would sometimes prefer to get as much stick time as the can, the FAA formally views autopilot as a safety device and recommends the maximum use of autopilots in congested airspace - particularly during the arrival and approach phases (not to mention holds). Autopilots are generally more accurate and reduce pilot workload, and as one might guess the workload gets higher and higher from descent to landing, 

 

Interestingly enough, FAA recommends that breakouts be performed manually (autopilot disengaged) in the case of breakouts from parallel approaches, but that has to do with speed of the breakout rather than accuracy.

 

There is a site that speaks about the use of autopilot in congested airspace, but my first few searches didn't come up with it.

This is more than likely an error in the navdata coding, not a bug in the airplane.

Thanks for all the good feedback !

 

Ok, since Jgid had the exact same issue this is most probably either a bug or navdata issue.

 

I checked the LFOP.txt file in FSX\PMDG\SIDSTAR folder, the line describing CEN N SID is correct, I don't know enough about how the navdata works to investigate more...

 

As per AP engagement, yes 400ft AGL is mentioned both in the FCTM and in the tutorial flights. I read in a Ryanair FCTM that if AP is not engaged by 1500ft then it should be mentioned in the departure briefing, so I gather the SOP is to have it on by 1500ft latest...

 

I tried again with HDG select and VOR/LOC armed, it works perfectly, so I guess I will do all my SID departures following that mode and then switch to LNAV upon reaching the transition, so as to minimize navdata issues...

 

Any thoughts on how to check/fix navdata ?

First off you should not be engaging the AP that low most real pilots fly it up to 10000 feet. Second is the first waypoint active in your FMS? Are you heading toward the active waypoint?

The aircraft can be handed over to the AP from 1500ft. In a few cases it is actually recommended to let the automagic handle the aircraft on departures/arrivals due to the high traffic density at certain airports, it's simply more precise. 

 

Not true affermations, while some real pilot could find very satisfing flying manual until 10000 ft and some others at 1500 or above this is not necessary and doesn't mean that a  "low altitude" A/P engaging pilot is wrost than a "10000 ft manual" pilot...simply sometimes are the same airline Sop's to require to engage A/P as soon as possible (from 400 ft AGL or from an altitude) to save fuel (also the best real world pilot cannot pilot so precisely like an autopilot does and so fuel burn is a little higher)...

Best Regards

Andrea B.

First off you should not be engaging the AP that low most real pilots fly it up to 10000 feet. Second is the first waypoint active in your FMS? Are you heading toward the active waypoint?

The aircraft can be handed over to the AP from 1500ft. In a few cases it is actually recommended to let the automagic handle the aircraft on departures/arrivals due to the high traffic density at certain airports, it's simply more precise.

 

Not true affermations, while some real pilot could find very satisfing flying manual until 10000 ft and some others at 1500 or above this is not necessary and doesn't mean that a "low altitude" A/P engaging pilot is wrost than a "10000 ft manual" pilot...simply sometimes are the same airline Sop's to require to engage A/P as soon as possible (from 400 ft AGL or from an altitude) to save fuel (also the best real world pilot cannot pilot so precisely like an autopilot does and so fuel burn is a little higher)...

Best Regards

Andrea B.

Please use the quote function so we know who you are replying to.

 

With regards to your comment, I have never heard of higher fuel burn in the hands of a pilot. I've simply been under the impression it has to do with workload management during critical phases of flight. I think the difference in fuel consumption would be marginal at best.