Trying to follow TCAS instructions...

[CaptainLars 21]

Well, two months later I think I did right. You can expect from an A/T to decrease speed in such a situation. I think that it's a bit less than perfectly programmed. Probably a bug. Of course, I have learned from this incident and am prepared now.

[skelsey 765]

3 hours ago, CaptainLars said:

Well, two months later I think I did right. You can expect from an A/T to decrease speed in such a situation. I think that it's a bit less than perfectly programmed. Probably a bug. Of course, I have learned from this incident and am prepared now.

It is absolutely, categorically not a bug, and I have to say I'm perplexed as to how you've reached that conclusion when several of us, including someone who flies the aeroplane for real, have  already explained to you that it's your technique at fault and not the aeroplane.

The AFDS is in THR REF | VNAV SPD for the climb. In this mode, you get climb thrust and pitch for the selected airspeed. If you do not follow the flight directors in manual flight, bad things happen. Why would you expect the A/T to reduce thrust when it's in a mode which gives you a fixed climb thrust?

As mentioned before, there is a reason why Boeing tell you to disconnect the A/T and A/P as a first response to a TCAS RA. You can try and second-guess the manufacturer's procedures if you like, but it generally doesn't end well because they usually have a good reason for telling you to do something! 

[Ephedrin 418]

Lars,

I think you fall victim to a logically fallacy, to be honest. No, not ment to sound rude.

Change your persective think about what exactly happens there:

the pilot has pressed the toga switches and told the airplane to set take off power.  The airplane is in TO mode which means it will give you take off thrust as calculated and set on the engine rating page in the FMS. As the flight directors (the brain of your a/c) are activated and vnav and lnav modes are armed it will also position the FD bars somewhere centered for lateral and somewhere around 15 deg nose up for vertical navigation. This is ment to be followed! Why? Because this is the pitch were your speed will match or reach the speed that is entered on your mode control panel in the SPD window. The airplane will do nothing else, this is simply the take off mode. It does not avoid mountains, trees or other airplanes. All it does is give you the correct pitch for your take off weight, config and thrust. If you don’t pitch up to match the FD, you will speed up and if you pitch up too much you will become too slow. 

Now you engage your autopilot and the airplane will soon change its mode to hold max climb power allowed as by your settings in the FMS and it will pitch down to match the speed restriction in the SPD window or the FMS as both are inputs for the flight computers, nothing else. Likely it will go for 250kts. Remember: throttle is set for climb power, pitch makes the speed. This is the mode that your aircraft is in. It can not do anything else as it wasn‘t designed to do anything else than this.

Now an other airplane crosses your way and the TCAS gives out an RA (resolution advisory) like decend, decend now! Normally your TCAS would communicate with the other plane‘s TCAS and I doubt yours would want you to decend when you are climbing with 3000fpm but this is a quite dumb PC simulator AI. Important about this is, that this RA is NOT coupled to your airplane‘s flight computers or autopilot system. But it is coupled to the speakers that alert you. If you now switch off your autopilot and push the nose down roughly the brain of your aircraft (the Flight director) is STILL in take off or climb mode, you have done nothing to change that. So it will keep setting climb power. It doesn’t know what you are doing and more important: it doesn’t care. To come back to idle thrust you have to switch off this stubborn system as you are the only one who is responsible of the controls who knows what is going on. And this means just every system that manipulates your controls: AP and AT. 

 

But (and this is probably the most important thing now): as this is something that is very likely never to happen in real life (tcas telling you to decend when your in initial climb) I personally use to completely ignore any RA in this phase of flight. Same on final. There MIGHT be something like this happening in real life that a C172 crosses your glide path or your initial climb, but in P3D most likely a 737 is sitting on your shoulders or landing the opposite direction. And I don‘t give a sh... about this AI behaviour. ;)

[Captain Kevin 586]

8 minutes ago, Ephedrin said:

There MIGHT be something like this happening in real life that a C172 crosses your glide path or your initial climb

Like this.

 

[CaptainLars 21]

20 hours ago, skelsey said:

The AFDS is in THR REF | VNAV SPD for the climb. In this mode, you get climb thrust and pitch for the selected airspeed. If you do not follow the flight directors in manual flight, bad things happen. Why would you expect the A/T to reduce thrust when it's in a mode which gives you a fixed climb thrust?

 

15 hours ago, Ephedrin said:

Lars,

I think you fall victim to a logically fallacy, to be honest. No, not ment to sound rude.

Change your persective think about what exactly happens there:

the pilot has pressed the toga switches and told the airplane to set take off power.  The airplane is in TO mode which means it will give you take off thrust as calculated and set on the engine rating page in the FMS. As the flight directors (the brain of your a/c) are activated and vnav and lnav modes are armed it will also position the FD bars somewhere centered for lateral and somewhere around 15 deg nose up for vertical navigation. This is ment to be followed! Why? Because this is the pitch were your speed will match or reach the speed that is entered on your mode control panel in the SPD window. The airplane will do nothing else, this is simply the take off mode. It does not avoid mountains, trees or other airplanes. All it does is give you the correct pitch for your take off weight, config and thrust. If you don’t pitch up to match the FD, you will speed up and if you pitch up too much you will become too slow. 

No offense taken, you are not sounding rude. I think I had or perhaps I still have  fragmentary knowledge of the Flight Director system and it's interaction with the A/T. I think that my faulty idea might come from the MS Default aircraft as they were in FSX. I started flightsimming with FS95 and continued with FS98, FS2000, FS2004 and FSX. Those birds were very simplistic and easy to handle. They had no VNAV or FLCHG, only VSPEED, and when you descended, the automatic reduced your thrust setting. Thank you for the very good explanation.

 

20 hours ago, skelsey said:

It is absolutely, categorically not a bug, and I have to say I'm perplexed as to how you've reached that conclusion when several of us, including someone who flies the aeroplane for real, have  already explained to you that it's your technique at fault and not the aeroplane.

I have reached this conclusion since myself and other people, who are, contrary to me, real world T7 Captains as per their own statement, have the impression that the pitch handling of this bird might be not exactly as it is in reality. Further, I have the purely subjective impression that the A/T has a tendency to set too much thrust, often more than needed (which can be a feature, and doesn't have to be a bug).

 

20 hours ago, skelsey said:

As mentioned before, there is a reason why Boeing tell you to disconnect the A/T and A/P as a first response to a TCAS RA. You can try and second-guess the manufacturer's procedures if you like, but it generally doesn't end well because they usually have a good reason for telling you to do something! 

I have searched the FCOM 1 and 2, the FCTM and the QRH and I did not find the line where it says that I should immediately disconnect A/P and A/T. It is logical to me to disconnect A/P, it just was not logical to me to disconnect A/T, too. Could you point me to the line where it says that you have to disconnect A/T?

[skelsey 765]

36 minutes ago, CaptainLars said:

I have searched the FCOM 1 and 2, the FCTM and the QRH and I did not find the line where it says that I should immediately disconnect A/P and A/T. It is logical to me to disconnect A/P, it just was not logical to me to disconnect A/T, too. Could you point me to the line where it says that you have to disconnect A/T?

Manoeuvres section of the QRH, "Traffic Avoidance":

Quote

Pilot Flying
If maneuvering is required, disengage the autopilot and disconnect the autothrottle. Smoothly adjust pitch and thrust to satisfy the RA command.
Follow the planned lateral flight path unless visual contact with the conflicting traffic requires other action.

 

36 minutes ago, CaptainLars said:

No offense taken, you are not sounding rude. I think I had or perhaps I still have  fragmentary knowledge of the Flight Director system and it's interaction with the A/T. I think that my faulty idea might come from the MS Default aircraft as they were in FSX. I started flightsimming with FS95 and continued with FS98, FS2000, FS2004 and FSX. Those birds were very simplistic and easy to handle. They had no VNAV or FLCHG, only VSPEED, and when you descended, the automatic reduced your thrust setting. Thank you for the very good explanation.

As you say, the issue stems from your understanding of the autothrottle system. The FCOM has more detail, but fundamentally the A/T has various modes. Some (like THR REF and IDLE) are fixed thrust modes, others (like SPD) are modes in which the A/T will modulate thrust in order to maintain a target speed. Because V/S mode is a "speed on thrust" mode, the autothrottle will revert to SPD mode as soon as it is selected. However, in a "speed on pitch" mode like VNAV SPD, or FLCH you will get a fixed thrust mode like THR or THR REF (or, conversely, IDLE in the descent). In these modes the AFDS targets a pitch to maintain speed and therefore if you fly away from the FD commands, as I say, Bad Things May Happen (like an overspeed if you under-pitch in a climb, or a stall if you over-pitch in descent with IDLE -- this is what happened to Asiana at SFO).

The really important thing to remember is that the A/P is just muscle. The FD is the "brains" and either the A/P follows it when it is engaged or you as the human follow it when the A/P is out -- your choice, but it is still the FD that is calling the shots as far as modes are concerned.

[CaptainLars 21]

6 hours ago, skelsey said:

Manoeuvres section of the QRH, "Traffic Avoidance":

Thanks, I had searched for "TCAS". 

 

6 hours ago, skelsey said:

As you say, the issue stems from your understanding of the autothrottle system. The FCOM has more detail, but fundamentally the A/T has various modes. Some (like THR REF and IDLE) are fixed thrust modes, others (like SPD) are modes in which the A/T will modulate thrust in order to maintain a target speed. Because V/S mode is a "speed on thrust" mode, the autothrottle will revert to SPD mode as soon as it is selected. However, in a "speed on pitch" mode like VNAV SPD, or FLCH you will get a fixed thrust mode like THR or THR REF (or, conversely, IDLE in the descent). In these modes the AFDS targets a pitch to maintain speed and therefore if you fly away from the FD commands, as I say, Bad Things May Happen (like an overspeed if you under-pitch in a climb, or a stall if you over-pitch in descent with IDLE -- this is what happened to Asiana at SFO).

The really important thing to remember is that the A/P is just muscle. The FD is the "brains" and either the A/P follows it when it is engaged or you as the human follow it when the A/P is out -- your choice, but it is still the FD that is calling the shots as far as modes are concerned.

What about HOLD? I figure that in VNAV or FLCH with A/T in HOLD, I control my rate of descent by manual throttle input. More thrust results in a less steep descent, and less thrust in a steeper descent because the aircraft is trying to meet the target speed with the thrust that I apply, am I right? When I initiate a descent, the aircraft first goes into IDLE and then in HOLD.

Thank you for the clarification about A/T and F/D, it's much appreciated.

[CaptainLars 21]

12 hours ago, 30K said:

 

Please read the forum rules, since you seem to be such an avid reader.

Btw, I never heard "Retard" in a Boeing...

Edited December 5, 2017 by n4gix
Removed quote of now hidden post.

[downscc 1,327]

18 hours ago, CaptainLars said:

What about HOLD? I figure that in VNAV or FLCH with A/T in HOLD, I control my rate of descent by manual throttle input. More thrust results in a less steep descent, and less thrust in a steeper descent because the aircraft is trying to meet the target speed with the thrust that I apply, am I right? When I initiate a descent, the aircraft first goes into IDLE and then in HOLD.

Correct, I use this technique often in the NGX and B744 but less so in the 777.  I prefer using the FPA to set up a controlled descent (I'll much prefer this to V/S, which is something I've been taught to avoid).  Many arrivals have a descent profile of about 2.4 - 2.8 deg and when you find that sweet spot it makes everything happen very smoothly.

[CaptainLars 21]

5 hours ago, downscc said:

Correct, I use this technique often in the NGX and B744 but less so in the 777.  I prefer using the FPA to set up a controlled descent (I'll much prefer this to V/S, which is something I've been taught to avoid).  Many arrivals have a descent profile of about 2.4 - 2.8 deg and when you find that sweet spot it makes everything happen very smoothly.

OK, thank you for your input.

[kevinh 409]

On 04/12/2017 at 5:05 PM, CaptainLars said:

I have reached this conclusion since myself and other people, who are, contrary to me, real world T7 Captains as per their own statement, have the impression that the pitch handling of this bird might be not exactly as it is in reality. Further, I have the purely subjective impression that the A/T has a tendency to set too much thrust, often more than needed (which can be a feature, and doesn't have to be a bug).

There is a pitch issue, which you need to keep in mind. PMDG tried to make pitch trimming more instinctive by reducing elevator effectiveness when out of trim, so you feel more control force to trim against. The more out of trim the less the effectiveness until it gets to zero. Unfortunately this has the side effect of reducing pitch control if you are out of trim. In AP control this isn't an issue as the aircraft is always in trim. When flying manually if airspeed changes and you don't trim to compensate then you will find yourself running out of pitch authority. In the real thing though you may may be pushing (or pulling) against trim the elevators remain effective.