AFDS: From TO/GA mode to ALT mode

[Hardy Heinlin]

Good morning,

I'm modifying the mode logic for the MCP altitude acquire phase during TO/GA. In the previous PSX versions, TO/GA only changes to ALT when the aircraft is within 50 ft of the MCP ALT; in other words, altitude acquire, which may start several hundred feet below MCP ALT depending on climb rate, is a submode of TO/GA. In the next version this will no longer be a submode of TO/GA but the actual ALT mode will engage instead. The ALT mode will acquire the MCP altitude (smoothly lower the nose) and then maintain it. This has the advantage that the pilot can select the SPD mode during the acquire phase already, and doesn't need to wait until reaching MCP ALT - 50 ft. During TO/GA you cannot change the THR mode to SPD. In ALT mode you can. So you can lower the nose with the SPD mode instead of the THR mode.

Well, this has a side effect. Your THR mode typically sets the power for a 2000 fpm climb rate, somewhere below the THR REF line. Now when altitude acquire starts in ALT mode, the 2000 fpm climb rate is not a target anymore. The airspeed will rise to max speed - 5 kt. Only at that point the THR mode will decrease the thrust. You can engage the THR mode manually when in ALT mode, and the thrust will increase to THR REF until max speed - 5 kt is reached. What should the THR mode do in ALT mode when the previous mode was TO/GA? It feels strange when the thrust increases as soon as ALT has engaged. Perhaps it should stay at the "2000 fpm" thrust setting in this case? (And accelerate to max speed - 5 kt with that "2000 fpm" thrust setting instead of the THR REF setting.)


Regards,

|-|ardy

[Hardy Heinlin]

I think I shouldn't forget that the MCP THR switch can only engage the THR REF mode, not the THR mode.

We can engage THR REF when in ALT, V/S or G/S.

Whenever we get THR while in ALT mode, it is always the remaining A/T mode of a previous TO/GA operation. Only the system can set this mode pair. The THR switch cannot do this.

When in ALT mode, I assume the THR mode will not increase the power. It will only decrease the power when overspeed is anticipated.

On the other hand, what would the THR mode do (while in ALT) when the airspeed decreases for some reason. It then has to increase the power. So I cannot simply inhibit the power increase command.


|-|

[skelsey]

On the other hand, what would the THR mode do (while in ALT) when the airspeed decreases for some reason. It then has to increase the power. So I cannot simply inhibit the power increase command.

Not based on anything other than surmising, but if the A/T mode is not SPD, why would the A/T 'know' to increase thrust to maintain a speed target in that situation?

In other words -- if we acknowledge that it would not reduce the thrust, why would we assume that it must increase thrust if IAS drops when it is not targeting any particular IAS?

[Hardy Heinlin]

We have one condition for the decrease-thrust command (no problem) which is:

• Overspeed anticipated.


We have two conditions for the increase-thrust command which are:

(1) • TO/GA changes to ALT, 2000 fpm phase is terminated
(2) • Airspeed in amber band, or any deceleration


If I want to inhibit the increase-thrust command in general, I have to ignore (1) and (2). If I want to inhibit it only for (1) in particular, the system needs to know how to tell (1) from (2).




In the current PSX versions, when in ALT mode, the THR mode is actually a hidden SPD mode whose hidden speed bug is set to max speed - 5 kt. It's the same as with THR REF while in ALT mode. Perhaps I shouldn't let the THR mode operate like the THRE REF mode. I guess the THR mode should set its hidden bug to amber band +5 when the airspeed is below that limit, and it should set it to max - 5 when overspeed is anticipated. For the range in between it should do nothing. The "hidden" SPD mode and the "hidden" bugs are necessary because otherwise it will oscillate: Acceleration out of low speed will end up at max speed, and the following thrust reduction will end up in low speed ... etc.

[Britjet]

Sorry Hardy, I don't know but I can try and look into it in the sim when I get chance..
Peter.

[Hardy Heinlin]

Thank you, Peter. It's solved now anyway. It has just been tested in a big sim (and in a CBT too): There is simply no THR mode with ALT mode. When ALT engages, THR mode changes to THR REF mode, and it does what the THR REF mode always does: It goes to the reference limit. (And includes overspeed protection as usual.)


Cheers,

|-|ardy

[Hardy Heinlin]

The modification is now available in PSX 10.55:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy