FLT DIR/PVD Selector

[MRFarhadi]

Hi,

I was experimenting with "Airspeed Unreliable" today and figured out something that I could not find enough references in the manuals to wrap my head around:
- Say we have identified the faulty ADC who putting out wrong data into the EIUs, MAWEAs, FCCs and such
- We switch (per QRH) the source selectors to healthy ADCs
- We also switch the FCCs to the healthy ones who are receiving the right data

Let's say the Captain is flying via the left seat while his instruments go Kaboom, he switches his ADC to the Right one (pun not intended). He'll also switch his FD to the Right FCC and orders the F/O to engage the Right Autopilot, putting the Right FCC into command. Now that both pilots are on ADC R and FCC R, SOURCE SEL F/D and SOURCE SEL ADC should come up on the EICAS.

My question is, per my experience with PSX, if, while still the Right A/P is engaged, the Captain selects his own Left FLT DIR on the Instrument Source Select Panel, the plane still would follow which of the 3 FCCs? Should the MCP selection of A/P decide which FCC is in command? or is it the Captain's Instrument Source Select Panel that determines which F/D the Autopilots follow?

(Clarification: I took off, having loaded "Training 704 - Takeoff D" situation present in the original PSX installation. Passing V1, Captain's airspeed indication freezes due to pitot blockage resulting in Stick Shaker activation and AIR SPEED LOW EICAS caution message. Switching Captain's ADC and FLT DIR to 'R' and engaging the Right Autopilot, airplane flies well-behaved. Once you select Captain's FLT DIR to C or L, while Right A/P is still engaged, the airplane's nose dives to achieve zero rate of climb to recover from the low airspeed condition. Same would apply if ADC selection is reverted to normal, also having the stick shaker activation alongside the airplane nose down input from the Right FCC)

Best,
Reza

[Hardy Heinlin]

Hi,

the ADC source selection logic for the autopilots is very, very complex. PSX uses hundreds of lines of code to model that logic. It involves multiple pairs of primary references and backup references. Each autopilot gets a dedicated pair, and these pairs are controlled by a stepper relay (energized by source selector actions) which sets the best possible configuration so that no pair uses the same setting.

The FCC/PVD source selector just sets the source for the onside PFD's FD bar movements and the PVD. It does not influence the autopilots. FD display and A/P servo control are two separate systems. There is even an airline option that removes the FD display when its source is equal to the source of the active A/P. UAL had this option. I guess it was used as a warning. The pilot should always use a different FCC for the FD on the PFD to crosscheck whether the commands agree with the other FCC's autopilot operation.

The onside ADC source selection also determines which ADC goes into the onside selected IRU. But this is just for this IRU's wind data computation which is sent to the FMS. As far as I recall, this IRU's ADC source is not used in the related A/P (each A/P is linked with one IRU).


Regards,

|-|ardy

[MRFarhadi]

Hi,

the ADC source selection logic for the autopilots is very, very complex. PSX uses hundreds of lines of code to model that logic. It involves multiple pairs of primary references and backup references. Each autopilot gets a dedicated pair, and these pairs are controlled by a stepper relay (energized by source selector actions) which sets the best possible configuration so that no pair uses the same setting.

The FCC/PVD source selector just sets the source for the onside PFD's FD bar movements and the PVD. It does not influence the autopilots. FD display and A/P servo control are two separate systems. There is even an airline option that removes the FD display when its source is equal to the source of the active A/P. UAL had this option. I guess it was used as a warning. The pilot should always use a different FCC for the FD on the PFD to crosscheck whether the commands agree with the other FCC's autopilot operation.

The onside ADC source selection also determines which ADC goes into the onside selected IRU. But this is just for this IRU's wind data computation which is sent to the FMS. As far as I recall, this IRU's ADC source is not used in the related A/P (each A/P is linked with one IRU).


Regards,

|-|ardy

My mistake was trying to get a grasp of the logic using VNAV. Now that I'm using FLCH SPD, I see what you meant by the "Complex" algorithm used to determine which ADC to use. Setting anything other than both FLT DIR selectors to the healthy side + ADCs to the healthy side results in VNAV levelling off with an amber dash on it on the FMA and the >AUTOPILOT EICAS message + amber cross line for the FLCH SPD on the FMA.

Thanks Hardy for the explanation.