LNAV and HDG SEL

[peb]

If the rudder is out of trim (in top LH corner of screen), will LNAV be able to follow the planned route or is the use of HDG SEL required?

[Hardy Heinlin]

"If the rudder is out of trim, will LNAV be able to follow the planned route or is the use of HDG SEL required?"

Let me split this into that:

Q1: "Does rudder trim disturb or help?"

Q2: "When the rudder generates a useless asymmetry, which mode can maintain a heading better -- LNAV or HDG SEL?"

Is there a longer story behind these questions?

I guess the answers require an explanation of the roll-yaw control on a jet:


Roll:

The human pilot and all AFDS roll modes except for the ATT mode use the aileron to acquire or maintain a specific heading. That specific heading may vary. But it's always a heading. Not a track, not a course, not an attitude -- but a heading.
TO/GA does aim at a track, but to maintain that track, the AFDS computes specific, situational headings.
LOC does aim at a localizer path, but to maintain that path, the AFDS computes specific, situational headings.
LNAV does aim at a route path, but to maintain that path, the AFDS FMC computes specific, situational headings.
At this point of situational heading computation, the above three roll modes operate in the same way as HDG SEL and HDG HOLD.
After this point, the computers turn the current heading command into an attitude command (roll) which is then sent to the FD display on the PFD and to the autopilot servo hydraulic controls.

Answer to Q2: It doesn't make much difference. When LNAV cannot follow the route anymore, you are in a non-normal side slip situation. This cannot be fixed by HDG SEL.


Yaw:

The human pilot and the autopilot should always try to keep the aircraft's yaw velocity at zero. You may consider three basic yaw "generators":

Thrust asymmetry
Structural asymmetry (e.g. aileron deflections, or damaged gear or flaps)
Rudder asymmetry (be it set by trim knob or directly by pedals doesn't matter)

The sum of these should always be zero. It's zero when your slip indicator on the PFD is centered. When it's not centered, the aircraft wants to turn away from the current heading. To stop this you need to correct your rudder (trim knob or pedal, doesn't matter; what counts are the rudder surfaces at the tail). If you don't stop it by rudder deflection, you can stop it by additional, opposite aileron -- which is limited and which cause a side slip situation. Side slips are applied by the autoland system. They shouldn't be applied in other flight phases. Side slip generates uneconomic drag and also constantly banks the passenger's coffee cup and the horses in the cargo.

Answer to Q1: The rudder surfaces at the tail should always be positioned so that the slip indicator is centered. When this rudder job is done correctly, it will help all roll modes. Otherwise it will disturb all roll modes.


Regards,

|-|ardy

[peb]

Thanks Hardy. I will have a look tomorrow when I am more 'with it' 🥳🥴
Happy Xmas

[peb]

Hardy

Thanks for your clear explanation - I now better understand how roll and yaw are handled.

The reason for the initial question is that I had a situation where the rudder was misaligned by the right pedal (as was shown in the indicator at the top LH corner of the screen).

I 'thought' that I observed that the FMS route could be followed better using HDG SEL rather than LNAV.

But I hve to admit that I didn't properly investigate and so my observations could be completely wrong or be due to my attempts at trimming.

I will repeat and with my new knowledge of how roll and yaw are handled investigate further.

Thanks again.

PS I have been involved in software and coding for ~40 years and I have to say that in all that time I have found only a few simulation packages where the software 'feels' as robust as your PSX. It's so nice to be able to be 99.99% sure that the software is behaving correctly when an unusual behaviour occurs, unlike some other packages!

You should be very proud of what you have produced. And the level of support you provide is exceptional.

[Hardy Heinlin]

Thank you for the kind comment.

The bank commands of the HDG SEL mode may be more rigorous than those of the LNAV mode -- or vice versa; it depends on their bank limit settings. The bank limit of HDG SEL can be set on the MCP which ranges from 5 to 25 and AUTO. The bank limit of LNAV is always automatic. When you set the HDG SEL's bank limit below 15 or so, you will see LNAV may be the more rigorous mode. For the automatic bank limit values you may refer to page 147 of the PSX manual. Note that these values are not used for side slip bank limits; so when you set 15, for example, you won't get a 15° side slip. The limits are used just for coordinated flight control. At lower limits the aileron control anticipates smaller deflections to avoid overshootings.


Regards,

|-|ardy

[peb]

Thanks again Hardy.