Alt Pitch Trim still work if all hyd sources got cut?
Thread Starter
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 233
Likes: 0
Received 0 Likes
on
0 Posts
Alt Pitch Trim still work if all hyd sources got cut?
Hi,
This is in relation to the B777. As FCOM 2 states,
"The alternate pitch trim levers are linked to the stabilizer trim control modules (STCM) via control cables, and then mechanically to the stabilizer."
"The center and right stabilizer cutout switches, located on the aisle stand, control hydraulic power to the respective stabilizer trim control module. Placing both switches in the CUTOUT position removes all hydraulic power from the stabilizer."
If STCM instructs hyd sources cutoff, there's still a mechanical link? So in the event L and R Hyd sources got cut off (STAB CUTOUT SWITCHES ARE PUT TO CUTOUT), Alt pitch trim levers can still control the stabilizer via cables? More importantly, does the STAB CUTOUT switches disable stab movement via mechanical link?
Can someone advise on this?
This is in relation to the B777. As FCOM 2 states,
"The alternate pitch trim levers are linked to the stabilizer trim control modules (STCM) via control cables, and then mechanically to the stabilizer."
"The center and right stabilizer cutout switches, located on the aisle stand, control hydraulic power to the respective stabilizer trim control module. Placing both switches in the CUTOUT position removes all hydraulic power from the stabilizer."
If STCM instructs hyd sources cutoff, there's still a mechanical link? So in the event L and R Hyd sources got cut off (STAB CUTOUT SWITCHES ARE PUT TO CUTOUT), Alt pitch trim levers can still control the stabilizer via cables? More importantly, does the STAB CUTOUT switches disable stab movement via mechanical link?
Can someone advise on this?
Last edited by extricate; 9th Sep 2017 at 05:04.
Join Date: Feb 2004
Location: Australia
Posts: 1,307
Likes: 0
Received 0 Likes
on
0 Posts
Only hydraulic power can move the stabiliser on the 777. If the switches are in CUTOUT, hydraulics are removed from the hydraulic motors in the tail. The Cutout switches close hydraulic shutoff valves electromechanically.
During alternate control, mechanical linkages (between the flight deck and the tail) operate "arm" and "control" hydraulic valves on the STCMs, but if CUTOUT is activated, no hydraulic pressure reaches these arm and control valves.
During alternate control, mechanical linkages (between the flight deck and the tail) operate "arm" and "control" hydraulic valves on the STCMs, but if CUTOUT is activated, no hydraulic pressure reaches these arm and control valves.
Thread Starter
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 233
Likes: 0
Received 0 Likes
on
0 Posts
Thanks for this. I always thought there's some form of direct mechanical linkages in case all systems fail
"In the normal flight control mode, when the stabilizer is manually shut down or failed, pitch trim is still available. Pilot pitch trim inputs change the PFC trim reference speed. The PFCs then reposition the elevators to trim the airplane."
"In the secondary and direct modes, both the primary pitch trim switches and the alternate pitch trim levers move the stabilizer directly. There is no trim reference speed."
Judging by the two statements above, if the system goes into Secondary mode, does moving the control wheel pitch trim switches move the stabilizer if the Stab Cutout is active? I suppose not cause the hyd sources for the stab is cutout. So if that's the case, what is the pitch mode in secondary or even direct mode if hyd cutout is active?
"In the normal flight control mode, when the stabilizer is manually shut down or failed, pitch trim is still available. Pilot pitch trim inputs change the PFC trim reference speed. The PFCs then reposition the elevators to trim the airplane."
"In the secondary and direct modes, both the primary pitch trim switches and the alternate pitch trim levers move the stabilizer directly. There is no trim reference speed."
Judging by the two statements above, if the system goes into Secondary mode, does moving the control wheel pitch trim switches move the stabilizer if the Stab Cutout is active? I suppose not cause the hyd sources for the stab is cutout. So if that's the case, what is the pitch mode in secondary or even direct mode if hyd cutout is active?
Join Date: Jun 2010
Location: NAT
Age: 40
Posts: 68
Likes: 0
Received 0 Likes
on
0 Posts
When in secondary and direct modes the control column deflects the elevator directly which is powered by the L HYD.
But in your scenario of having both stab cutout switches to cutout, and also being in secondary mode, I am not sure what kind of trim is available (unless the altn pitch reverts to some purely mechanical mode bypassing the STCM).
Check this link, it suggests that the altn pitch trim is controlled via cables if need it be.
https://www.google.ae/search?q=b777+...mKkJ86SXVIwhM:
Cheers
But in your scenario of having both stab cutout switches to cutout, and also being in secondary mode, I am not sure what kind of trim is available (unless the altn pitch reverts to some purely mechanical mode bypassing the STCM).
Check this link, it suggests that the altn pitch trim is controlled via cables if need it be.
https://www.google.ae/search?q=b777+...mKkJ86SXVIwhM:
Cheers
Last edited by Capt. Flamingo; 12th Sep 2017 at 17:42.
Thread Starter
Join Date: Apr 2008
Location: Test
Age: 35
Posts: 233
Likes: 0
Received 0 Likes
on
0 Posts
Thanks for this.
Open to views, i'm sure someone will know more. Although the chances of all 3 hyd fail are rare, but just in case we need mechanical control for pitch, is it available from the Alt Pitch Trim?
Open to views, i'm sure someone will know more. Although the chances of all 3 hyd fail are rare, but just in case we need mechanical control for pitch, is it available from the Alt Pitch Trim?
