Caravan Pilots and Mechanics

PLEASE NOTE: You are currently viewing the OLD FORUMS which are working in archived mode. See our NEW FORUMS at this link!

Caravan Pilots

Caravan Pilots from around the World
It is currently Mon Dec 18, 2017 9:56 am

All times are UTC [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
 Post subject: KFC-225 pitch trim issues
PostPosted: Mon Oct 12, 2009 9:12 am 
Offline

Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
I have a friends that is having some Caravan AP issues with regards to Pitch Trim/Alt Hold in flight, but the AP self tests do not indicate any errors with the P/T system. I wonder if anyone has been through this before and can give some pointers as to where to look for the problematic components. Thanks,

Equipment details:

Autopilot
Manufacture Honeywell Bendix/King
Model no. KFC-225

Altimeter:
Manufacture Bendix king, Honeywell
Model no. ALT KEA 130A

Attitude indicator:
Manufacture BENDIX KING ,HONEYWELL
Model no. KI 256 FLIGHT COMMAND IND

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

Altitude Hold issues:
1.During climb or descend, Altitude mode did not capture the pre-selected altitude (armed altitued). Eventhough the autopilot annunciator panel has shown "CAP" (altitude capture) but the aircraft continue climbing or descending pass the pre-selected altitude.

2.In level flight, When "Altitude hold" is selected at 5,000 ft and the aircraft altitude varies due to 200ft+ updraft/downdrafts, the autopilot doesn't correct to the selected altitude.

3.Flight director test: altitude preselect test on ground.

· Selected an altitude 500' above field elevation

· Turn the baro setting on the altimeter to match the altitude preselect setting

· The flight director moves from climb indication to level flight indication.

. AP annunciator did not respond to the above, thus not changing "Alt Arm" to "Captured".

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

Electric pitch trim

1. During a prolonged AP turn in flight, the AP trims nose up. Also, the AP doesn't compensate for airspeed variations to maintain the altitude resulting in gain/loss unchecked by AP.

2. With autopilot engaged on the ground, the autopilot trims opposite the direction of the control wheel movement (Push control wheel - AP trims nose up. Pulling control wheel - AP trims nose down.


Any thoughts on what could be the issue or how to isolate the component that needs servicing?


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Mon Oct 12, 2009 11:04 am 
Offline

Joined: Sat Oct 13, 2007 5:29 am
Posts: 8
I haven't used the 225, but have flown with the 150.

Your description of the electric pitch trim operation on the ground sounds normal for the following reasons:
1 - In a turn increased lift is required to maintain altitude at the same airspeed. Aft elevator input will be required - the trim system will add nose up trim to reduce load on pitch servos. When you stop turning the opposite should happen and the trim system should add nose down trim.
2 - If you pull aft on the control column on the ground the pitch servos will try and push it forward. The trim system will then add nose down trim to try to reduce the load on the pitch servos. If you push forward on the control column the pitch servos will try and move it aft. The trim system will the add nose up trim to try to reduce the load on the pitch servos.

My suggestion (for what it's worth) is to engage the autopilot in pitch hold mode in level flight (trim the aircraft first). Use the pitch nose up button to to select a nose up attitude and make sure it maintains the new attitude. Make sure that nose up trim is wound on automatically. Press the autopilot disconnect button without pushing or pulling on the controls and there should be no pitch change. Reengage the autopilot in pitch hold mode in level flight. Then use the pitch nose down button to select a nose down attitude and make sure it maintains the set attitude. Make sure that nose down trim winds on automatically. Press the autopilot disconnect button without pushing or pulling on the controls and there should be no pitch change. If this tests OK it rules out problems with the mechanical parts of the autopilot.

Two questions:
When these altitude excursions are occurring what is the flight director indicating?
Is it consistent or intermittent?


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Wed Oct 14, 2009 6:24 pm 
Offline

Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
Thanks for your reply,

I am waiting for the result confirmations and will post them shortly. My friend couldn't create a user account for this forum so I'm relaying the info for him.


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Mon Oct 19, 2009 11:58 am 
Offline

Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
below are the questions (*) and my friend's replies. If anyone can give a a direction on where to begin the troubleshooting it would be most helpful. Maintenance has cleaned the connectors on all systems, but are having difficulty pinpointing the source of the trouble.

Thanks,


* so does this mean that when you are flying the AP doesn't capture altitude at all?
Yes, AP doesn't capture altitude. Even though AP computer annunciator shows "CAP".

* If you put the AP in PT mode, you can not use the up down buttons to electrically move the trim wheel from the AP computer?
Yes, pilots can not use the up/down buttons to electrically move the trim wheel from AP computer.

* How about the left pilot switch?
Left pilot can control the electric pitch trim using the yoke switch.

* If you are flying at 5,000 and select 7000ft, the AP won't do anything to climb when engaged?
Yes, AP doesn't do anything. There's no movement of electric pitch trim. There's only the AP annunciator showing "CAP" when reaching altitude manually.

* Just to clarify, on approach mode is the HSI and FD are following the approach smoothly but the AP doesn't follow the glide?
Yes, on "APR" and "HDG" mode HSI and FD are following the approach smoothly, but the AP doesn't follow the glide slope.


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Mon Dec 14, 2009 12:44 pm 
Offline

Joined: Fri Aug 15, 2008 10:43 am
Posts: 14
Location: Thailand
So it turned out to be the pitch trim servo itself that caused the problem. Probably sending bad feedback to the computer and throwing the whole system for a loop.

Thanks to all that looked into this,


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Wed Jan 13, 2010 4:21 pm 
Offline

Joined: Fri Feb 08, 2008 4:46 pm
Posts: 1
There is a Honeywell service memo 292 that i employee avionics to carry out every 12 months or 1 year.
There are many issues with the system. If the pilots don't trim the system first before engagement there is going to be problems.


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Fri Aug 06, 2010 10:16 pm 
Offline

Joined: Tue May 17, 2005 3:09 am
Posts: 4
Lubricate the piano hinge on the trim tab and do ti often it helps with this


 
 Profile  
 
 Post subject: Re: KFC-225 pitch trim issues
PostPosted: Sun Aug 08, 2010 12:42 am 
Offline

Joined: Tue May 17, 2005 3:09 am
Posts: 4
wanted to add that if the trim works fine on the ground and it is stiff the air this is normally the actuator in the horizontal stabilizer gone bad. this will always mess up the AP as it hits the limit on the pressure


 
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2002, 2006 phpBB Group
[ Time : 0.044s | 14 Queries | GZIP : Off ]