Originally Posted by
TheBusFlyer
During climb, if it can’t reach the target altitude in 125 seconds, it sets climb thrust, displayed as THR REF on FMA.
Any insights during descend ?
I have never seen it go IDLE - FLCH SPD. Even when it can’t reach it in 125 sec, FMA shows THR but actual thrust is at Idle.
Why so? IDLE - VNAV SPD/PATH exists. Why not display IDLE for FLCH when it’s really setting Idle thrust ??
I am aware it then transitions to HOLD but my question refers to the first AT mode displayed (THR) as we select FLCH during descent.
I started preparing the following a while ago, and I think a few of the posts in the interim should have clarified things a little for you, but I will post this for you anyway.
With regard to this last post of yours. When on descent, when thrust is reduced to idle, it is my recollection that FMA transitions to HOLD pretty much simultaneously with idle stops on the levers.
But in reality does it matter when it transitions? Is it going to lead you to change your inputs if doesn't tell you what you expect? Are you going to close the already closed thrust levers? Are you going to adopt some alternate procedure to achieve what you have already asked the AFCS to do for you?
pre prepred respose to earlier.
Consider these points;
1) If FLCH
controls V/s, why then do we need a separate and distinct VS mode. On your reading, they are doing exactly the same thing?
2) Why can you NOT select FLCH and VS modes simultaneously?
I have no source for my posit, as I am the owner of a fried motherboard that has locked away all my previously owned manuals and documentation. My comments are based on considerable observation of the way things work, over a lengthy period of service on type. I have not been in a control seat for over 6 years, so apologies for any holes in my recollection.
Request for confirmation can run both ways. Can you produce a credible reference for your position that FLCH will calculate and target a specific ROC/ROD in compliance with your assumption of method of operation. Or Vessbots for that matter.
The fairy story below may help develop a perspective of how it works.
******************
Hi my name is FLCH. I live in a box in the E&E. Living with me are multiple modes, VS, VNAV to name a couple. We each have different roles, sometimes utilising different parameters that are pertinent to our allotted tasks. We normally do not all work together at the same time, but we harmonise to produce the required outcomes. Our environment is populated with PFC’s ACE’s and a plethora of electronic gizmos and sensors, who help us keep in touch with all the parameters and environmental factors we encounter in our daily work. Some call our home PFM, which roughly translates to Pure Magic and refers to our output.
Our role is to take instructions from our bosses who sit in the comfy chairs upstairs. We task allocate then analyse the requests/instructions. We formulate a response, translate them into electronic form and communicate them to the components on the outer extremities of our environment, with a view to accommodate the bosses wishes. We are each and collectively obedient servants. At all times we verify the effect of our actions and relay important status info to our bosses
As an example: If someone upstairs presses my call button (FLCH) I will instantly jump into action. I am not the smartest kid in the room but I do well that which is asked of me. Some wrongly endow me with credit for calculations far more sophisticated than those for which I am qualified. I call them overthinkers. Sometimes those overthinkers get frustrated with me because I do not behave the way they think I should. I will behave the way I am programmed at build, in compliance with the instructions I have received and within the programmed limitations of the combined systems.
When those in the comfy seats upstairs ask me to climb or descend, I will adjust the output of the noise makers, appropriately. Those upstairs will tell me what speed they want to maintain (lock) throughout, and also tell me what level they want to go to.
Go up - increase noise
Go down - decrease noise
As I reduce or increase the noise and maintain the locked speed, I will constantly monitor the
consequential variation in vertical speed and calculate the approximate time to level. If that time is 2 minutes from my call button press, or less, I will cease reducing/increasing noise. If that 2 minute time window is unachievable, it will be ignored and I will continue the movement of the noise levers until they reach their limits. Failure to comply with the time restriction is no biggie, it was only an arbitrary thing anyway. Just to keep everyone in the loop, I will let the supervisor, my cousin FMA, know that I have stopped fiddling with the noise makers. In the case of descent he will then put out a HOLD message. That is to let those upstairs know that I have done my bit and gone for coffee, if any variation of noise level is required /desired, they are going to have to do it themselves, or call on one of my other cousins to have some input. Cousin VS could step in but he and I cannot work together so to maintain the harmony I will step out. Regarding the HOLD message I prefer to think of it as an instruction to those upstairs to hold on to the noise levers because if, in their infinite wisdom, they decide some variation in profile is required, they are now in charge and any variation is on their head (or more precisely, in their hands). I am happy to sit and watch, and I am here if they call but I will not interfere. My job is now done.
I will sit back with my coffee and wait for cousin Capture, to take over. When he comes in he will resume responsibility for noise level and the attitude to maintain the requested speed and altitude. So, I am simple, but I am good at my job.
PFM.
As operational footnotes.
1) For short level change, cabin comfort will be enhanced by the use of VS rather than FLCH due to the smoother and lesser application of the noise levers.
2) To see what actually goes on when you press FLCH, pull up FCTL on one of your MFD’s and then set up a level change. May not give much value in the sim but could be enlightening in real life. Don’t know, haven’t done it. It would also be accessible through the maintenance terminal, but that is a complex beast to get into when unfamiliar.