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jules744

[Technical] Trim reference for speed

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Good evening everyone!
 

 


  • I have flown the 777 (PMDG) for couple months. As I come from the 737, I still a bit skeptical about how the trimming should be performed. So far, I have read FCOM's, I have checked on YouTube videos about it, also I have checked across PPRuNe, but I have not been able to really understand the logic which is behind.

 

  • Statement: I understood that on 777, pushing the trim switch located on the yoke does not make the stab moving directly -- as it does on 737 --. Instead, it changes the trim reference speed, the speed which is targeted by the current configuration by the trim. For instance : I am at FL080, and I want to descent and keeping the speed, so I just reduce the throttle and push forward. Now I want to make the same but I want to reduce the speed then I push forward and trim downwards -- because I want the reference speed to be lower!-- at the same time (seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...), so in this situation I don't know what to do...
  • If someone who knows the aircraft could explain me the correct way of using the trim reference speed, I would be very happy.

 

Have great flights ! smile.gif

Thanks !!

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Good evening everyone!

 

 

  • I have flown the 777 (PMDG) for couple months. As I come from the 737, I still a bit skeptical about how the trimming should be performed. So far, I have read FCOM's, I have checked on YouTube videos about it, also I have checked across PPRuNe, but I have not been able to really understand the logic which is behind.

 

  • Statement: I understood that on 777, pushing the trim switch located on the yoke does not make the stab moving directly -- as it does on 737 --. Instead, it changes the trim reference speed, the speed which is targeted by the current configuration by the trim. For instance : I am at FL080, and I want to descent and keeping the speed, so I just reduce the throttle and push forward. Now I want to make the same but I want to reduce the speed then I push forward and trim downwards -- because I want the reference speed to be lower!-- at the same time (seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...), so in this situation I don't know what to do...
  • If someone who knows the aircraft could explain me the correct way of using the trim reference speed, I would be very happy.

 

Have great flights ! smile.gif

Thanks !!

 

 

http://www.theairlinepilots.com/forum/viewtopic.php?t=739

 

Pitch Trim Switches 

 

• in the normal mode in flight, changes the trim reference airspeed. 

 

• in the normal mode on the ground, moves the stabilizer 

 

• in the secondary and direct modes, moves the stabilizer. 

 

The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes. 

 

 

Alternate Pitch Trim 

 

The levers move the trim reference airspeed (normal mode) and also move the stabilizer (all modes). 

 

The alternate pitch trim levers are linked to the stabilizer trim control modules (STCM) via control cables, and then mechanically to the stabilizer. 

 

Alternate pitch trim commands have priority over wheel pitch trim commands in all flight control modes. 

 

Moving the alternate pitch trim levers with the autopilot engaged does not disconnect the autopilot, but does move the stabilizer. 

 

Moving the alternate pitch trim levers during stall or overspeed protection does move the stabilizer, but does not remove column forces. 

 

Note: The alternate pitch trim levers should not be used with the autopilot engaged, or during stall or overspeed protection. 

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(seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...)

 

Nope. That's still trim down. Sure, you're grabbing the bottom of the trim wheel and moving your arm up, but the action is called trimming down. It's not the motion of your arm, or the wheel, or the button - it's the action of what it's doing to the nose. In a Cessna, rolling the trim wheel from bottom, up to the top, is still trimming down, because the effect of your action is to move the nose down. You'll note that the Cessna trim wheel doesn't refer to "Trim Up" or "Trim Down," either. It references "Nose Up" and "Nose Down" in an effort to make it a little clearer.

 

Here's a picture of the little bug smasher I'm taking out in a couple hours:

hi-172DR-P.JPG

(Tough to see, but it definitely refers to what effect it will have on the nose, regardless of the motion on the wheel.)

 

Regardless:

TRIM UP is to reduce the trim reference speed.

TRIM DOWN is to increase the trim reference speed.

 

From the sound of it, you fly small planes. If so, your instructor has probably mentioned "pitch plus power equals speed." Assuming you have full power in, you can set a certain pitch to climb out at a constant 70 knots. If you lower the nose some, then you can climb out at a constant 80 knots. The only difference here (from 70 to 80) is that I trimmed the nose down, which increased the speed at which I'm climbing.

 

The same is true for a descent. Assuming a constant power setting, if I am descending at 80 knots and I want to descend at 90 knots - without changing the throttle - I simply need to lower the nose. In order to do this, I will trim down. Trimming down, again, increases the speed.

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Nope. That's still trim down. Sure, you're grabbing the bottom of the trim wheel and moving your arm up, but the action is called trimming down. It's not the motion of your arm, or the wheel, or the button - it's the action of what it's doing to the nose. In a Cessna, rolling the trim wheel from bottom, up to the top, is still trimming down, because the effect of your action is to move the nose down. You'll note that the Cessna trim wheel doesn't refer to "Trim Up" or "Trim Down," either. It references "Nose Up" and "Nose Down" in an effort to make it a little clearer.

 

Here's a picture of the little bug smasher I'm taking out in a couple hours:

hi-172DR-P.JPG

(Tough to see, but it definitely refers to what effect it will have on the nose, regardless of the motion on the wheel.)

 

Regardless:

TRIM UP is to reduce the trim reference speed.

TRIM DOWN is to increase the trim reference speed.

 

From the sound of it, you fly small planes. If so, your instructor has probably mentioned "pitch plus power equals speed." Assuming you have full power in, you can set a certain pitch to climb out at a constant 70 knots. If you lower the nose some, then you can climb out at a constant 80 knots. The only difference here (from 70 to 80) is that I trimmed the nose down, which increased the speed at which I'm climbing.

 

The same is true for a descent. Assuming a constant power setting, if I am descending at 80 knots and I want to descend at 90 knots - without changing the throttle - I simply need to lower the nose. In order to do this, I will trim down. Trimming down, again, increases the speed.

 

Hi!

 

Thanks for your reply, it is the exact type of anwser I sought !

I see now, the point is that I was too much thinking like  C172-trimining whereas it is far different, actually!

 

------------------------------------------------------------------------------

When you say trim up you mean pushing down on the switch to increase trim ?

------------------------------------------------------------------------------

 

Cheerio!

http://www.theairlinepilots.com/forum/viewtopic.php?t=739

 

Pitch Trim Switches 

 

• in the normal mode in flight, changes the trim reference airspeed. 

 

• in the normal mode on the ground, moves the stabilizer 

 

• in the secondary and direct modes, moves the stabilizer. 

 

The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes. 

 

 

Alternate Pitch Trim 

 

The levers move the trim reference airspeed (normal mode) and also move the stabilizer (all modes). 

 

The alternate pitch trim levers are linked to the stabilizer trim control modules (STCM) via control cables, and then mechanically to the stabilizer. 

 

Alternate pitch trim commands have priority over wheel pitch trim commands in all flight control modes. 

 

Moving the alternate pitch trim levers with the autopilot engaged does not disconnect the autopilot, but does move the stabilizer. 

 

Moving the alternate pitch trim levers during stall or overspeed protection does move the stabilizer, but does not remove column forces. 

 

Note: The alternate pitch trim levers should not be used with the autopilot engaged, or during stall or overspeed protection. 

Thanks for the explaination too, very usefull, got a great deal of readings for these holidays ! :)

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I see now, the point is that I was too much thinking like  C172-trimining whereas it is far different, actually!

 

No -- you're over-thinking it slightly -- it's designed to feel almost exactly like trimming a C172.

 

On the control column on the 777 (and indeed the 737 and 747) the trim switches work in exactly the same way. If you want to select a lower nose attitude, you push forward on the control column. You then click the trim switches "up" (towards the top of the control column) to select nose down trim, which relieves the pressure on the controls. It's more intuitive than it sounds!

 

The 777 is, from the point of view of how it feels, exactly the same: you push forward on the control column and push the trim switches "up" (towards the top of the control column) to relieve the pressure.

 

Most (if not all) conventional aircraft are speed-stable, the C172 included. To prove it, take the default Cessna up and trim it so it's flying straight and level at 3000ft, at 2200rpm: the airspeed should settle at around 90kias and you should be able to take your hands off the controls without the aircraft wanting to leave this state.

 

Now take your hands and feet off the controls and pull the throttle back to 1700rpm. The aircraft will pitch down (initially partially because of the thrust vector) and start a gentle descent, whilst the airspeed remains roughly unchanged at around 90kias.

 

Likewise, if you do the same thing but instead increase to full power, the aircraft will start to climb but the speed will, more or less, remain at 90kias.

 

The 777 is designed in exactly the same way, except you're trimming through the FBW computers which gives you a little more precision and corrects for things like the thrust-pitch couple you get with underslung engines, flap extension etc. For this reason, the trim switches select a speed rather than a stabiliser position: because the FBW may decide to move the stabiliser to counteract configuration changes. However, from a pilot's point of view it should fly exactly the same as any other conventional aircraft.

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Yup, I never set the trim by the trim ref speed, always by whatever it takes to neutralize pressure on the control. You can safely fly this plane with the trim ref speed bug off, as it is an option and not all operators use it.  I use it as just an additional piece of information.

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Yup, I never set the trim by the trim ref speed, always by whatever it takes to neutralize pressure on the control. You can safely fly this plane with the trim ref speed bug off, as it is an option and not all operators use it. I use it as just an additional piece of information.

The trim reference speed bug is a PMDG option for the sim. The real aircraft does not have the option.

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That's true; I had to look back at the Introduction and first clue that this is true is that the option is in the simulation section along with other "simulator only" features. Thanks for pointing that out Kevin.

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So the trim reference speed is essential the same as moving the speedknob and selecting speed intervene or/and FLCH on the mcp? what do i not understand or miss?

 

Thanks Michael Moe

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You set the speed on the speed bug and while autopilot is disabled, slowly push the trim switch up or down, depending if you want to increase or decrease speed. 1 second is 10 knots.

 

If you watch that EVA Air video I attached above, it shows you everything you need to know about Trim Reference Speed.

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You set the speed on the speed bug and while autopilot is disabled, slowly push the trim switch up or down, depending if you want to increase or decrease speed. 1 second is 10 knots.

 

If you watch that EVA Air video I attached above, it shows you everything you need to know about Trim Reference Speed.

 

Thanks and did see the video.

 

Autopilot off. Autothrottle on.

 

Michael Moe

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