Trying to understand FD behavior

[bpcw0001]

Hi,

after flying the Challenger for a while now, I keep wondering about the behavior of the flight director. The format does not matter, i. e. X-shape or V-shape is irrelevant, I tried with both.

Scenario:

On an approach (ILS for example), hand-flying, and drifting slightly of course laterally (like 1/4 dot). The deviation clearly shows on the rose CDI needle, but the FD does not command any course correction. According to the FD, I am perfectly on course. If I flew the FD only, I'd end up next to the runway!
The FD commands a correction only when the deviation gets larger.

I don't know if that is a limitation of screen resolution that indicating small corrections would not translate into a visible change or if this is how the system generally works.

Any insights for me here?

 

[JRBarrett]

On 7/10/2023 at 10:16 AM, bpcw0001 said:

Hi,

after flying the Challenger for a while now, I keep wondering about the behavior of the flight director. The format does not matter, i. e. X-shape or V-shape is irrelevant, I tried with both.

Scenario:

On an approach (ILS for example), hand-flying, and drifting slightly of course laterally (like 1/4 dot). The deviation clearly shows on the rose CDI needle, but the FD does not command any course correction. According to the FD, I am perfectly on course. If I flew the FD only, I'd end up next to the runway!
The FD commands a correction only when the deviation gets larger.

I don't know if that is a limitation of screen resolution that indicating small corrections would not translate into a visible change or if this is how the system generally works.

Any insights for me here?

 

Sorry for the late response.

I am a mechanic and avionics technician on r/w CRJ-200s which have very similar systems to the CL650, although the CRJ uses the older Collins Proline 4. rather than the newer Proline 21 on the 650. But, the operating principles are the same.

The flight director shows steering commands coming from the autopilot.

However, when the aircraft is flying an ILS approach in “green needles” the steering commands for both the pitch and roll servos are not coming from the autopilot’s command logic. Instead, the aircraft (when the autopilot is engaged in approach mode) is being steered directly by localizer and glideslope deviation outputs of the VIR-432 Nav radio. In this mode, the autopilot flight director is only in a passive “monitoring” state. The Nav radio itself is “doing the driving”.

The Nav radio responds much more quickly to any deviation from the localizer or glideslope than the flight director - especially in the latter stages of the approach.

In fact, on some Boeing airliners, the SOP is to turn the flight director completely off once established on an ILS, as the flight director command bars can be distracting in this scenario because of their slower response to any deviation from the localizer or glideslope.

Edited August 9, 2023 by JRBarrett

[bpcw0001]

On 8/9/2023 at 6:22 AM, JRBarrett said:

Sorry for the late response.

I am a mechanic and avionics technician on r/w CRJ-200s which have very similar systems to the CL650, although the CRJ uses the older Collins Proline 4. rather than the newer Proline 21 on the 650. But, the operating principles are the same.

The flight director shows steering commands coming from the autopilot.

However, when the aircraft is flying an ILS approach in “green needles” the steering commands for both the pitch and roll servos are not coming from the autopilot’s command logic. Instead, the aircraft (when the autopilot is engaged in approach mode) is being steered directly by localizer and glideslope deviation outputs of the VIR-432 Nav radio. In this mode, the autopilot flight director is only in a passive “monitoring” state. The Nav radio itself is “doing the driving”.

The Nav radio responds much more quickly to any deviation from the localizer or glideslope than the flight director - especially in the latter stages of the approach.

In fact, on some Boeing airliners, the SOP is to turn the flight director completely off once established on an ILS, as the flight director command bars can be distracting in this scenario because of their slower response to any deviation from the localizer or glideslope.

Wow. Thanks for this comprehensive explanation. 

And a bloody good simulation from Hot Start if they took this into account.

This would basically render the FD useless for ILS approaches, at least for lateral tracking, and we have to do them "raw data" just flying the needles.

Edited August 15, 2023 by bpcw0001

[JRBarrett]

6 hours ago, bpcw0001 said:

Wow. Thanks for this comprehensive explanation. 

And a bloody good simulation from Hot Start if they took this into account.

This would basically render the FD useless for ILS approaches, at least for lateral tracking, and we have to do them "raw data" just flying the needles.

Yes, on an ILS approach, always prioritize the green needles over the flight director.

[richjb]

Ok... we need correct something here.   We do not turn off the flight directors when established on an ILS approach, even in the Boeing aircraft.  In fact, without a flight director, in part 121 or part 135 operations, your approach landing minimums increase automatically to 4000 RVR or 3/4 SM regardless of what is on the instrument approach by OpSpec.   OpSpec requires use of the flight director or autopilot to use the published landing minimums for these operators.  In addition, the use of flight director, autopilot, or HUD is required in certain circumstances to fly an approach to 1800 RVR instead of the published 2400 RVR as noted on the approach.  

Speaking to Collins about the Proline 21 flight director's behavior on an ILS approach, the flight director command bars do follow with an initial pitch change that you may make manually. Not with the roll, but with pitch only.  Until it sees a deviation from the glideslope, the command bars will not display a correction command.  For this reason, it appears that the command bars initially follow any pitch change initiated by the pilot, intentional or otherwise.  I have tested this both in the simulator and the actual aircraft.  It is a behavior that we see in other autoflight systems as well, for example, the Garmin system in the Lear 75.  For the most, this behavior is accurately depicted in HS650. 

The flight director is a required component for ILS approaches in a turbojet.  It should not be disregarded when flying an ILS approach. 

 

Rich Boll 

 

[bpcw0001]

On 9/7/2023 at 7:13 PM, richjb said:

Ok... we need correct something here.   We do not turn off the flight directors when established on an ILS approach, even in the Boeing aircraft.  In fact, without a flight director, in part 121 or part 135 operations, your approach landing minimums increase automatically to 4000 RVR or 3/4 SM regardless of what is on the instrument approach by OpSpec.   OpSpec requires use of the flight director or autopilot to use the published landing minimums for these operators.  In addition, the use of flight director, autopilot, or HUD is required in certain circumstances to fly an approach to 1800 RVR instead of the published 2400 RVR as noted on the approach.  

Speaking to Collins about the Proline 21 flight director's behavior on an ILS approach, the flight director command bars do follow with an initial pitch change that you may make manually. Not with the roll, but with pitch only.  Until it sees a deviation from the glideslope, the command bars will not display a correction command.  For this reason, it appears that the command bars initially follow any pitch change initiated by the pilot, intentional or otherwise.  I have tested this both in the simulator and the actual aircraft.  It is a behavior that we see in other autoflight systems as well, for example, the Garmin system in the Lear 75.  For the most, this behavior is accurately depicted in HS650. 

The flight director is a required component for ILS approaches in a turbojet.  It should not be disregarded when flying an ILS approach. 

 

Rich Boll 

 

Thanks for the insights. I had already been wondering as to why have an FD (even mandated by regulations) if that thing cannot provide precise guidance during precision approaches.
My observation is that in the HS Challenger, the flight director reacts quite promptly to glide slope deviations, but appears relatively unprecise when it comes to lateral deviation.
The same goes for the X-bar display in the HS challenger. Like I said, if I flew only the FD in the HS challenger with respect to *lateral* deviation, I might end up left or right next to the runway. The "green needle" on the rose though provides accurate lateral deviation information. That's what I find puzzling. Why does the green needle deviation not seem to translate into lateral FD commands?

Edited September 28, 2023 by bpcw0001