SPZ-7600 ILS problem

[karote]

Hi All,

We are encountering a problem when flying ILS approach. The glideslope deviation fluctuates randomly; for a while the R/C is exactly on the correct glidepath and then suddenly the R/C starts to deviate and the corrective action seems to be really slow, althought the pitch command indicator on ADI shows correct attempt to restore the optimum path. Flying several ILS approaches in sequence does not reveal any regular pattern, but sometimes we are within 0.5 dot through the whole glide and then sometimes more than 1 dot off especially under 500 ft where the glide path gets narrower. This R/C has dual FD, 3-axis, SPZ-7600 autopilots.

The ILS signal is received correctly, without flags. Pilots say that with reference to PAPI the GS indication on EHSI is correct.The R/C performs autolevelling correctly, so the radar altimeter is functioning for GS gain programming.
The airdata system has some small fluctuation in vertical rate (inside 25ft per minute) and the autopilot calculated TAS is slightly smaller than expected (2-4 kts), but still these errors are quite small and not considered as causing this issue completely. Basically, it feels that the autopilot is just slow on pitch channel!

Does anyone know what kind of registers there are that could be used to monitor autopilot pitch channel gain? Are there any adjustable gain settings on pitch channel or some adjustable pitch rate settings?
How to test that the pitch jackscrew actuator has the needed speed&response to keep the R/C at correct glideslope?

Thank you in advance,
Karote

[Sir Korsky]

Had similar situation once and a bad servo was deemed to be the problem.

[[email protected]]

With a 3 axis AP you need to help it by reducing power at the top of drop - all the AP can do is bring the nose up or down to maintain the glidepath so you will see changes in IAS if you don't set the correct power.

[havick]

Assuming you’re using the correct technique, is it just the one airframe doing it?

Are you only flying the same ILS to the same runway every time? Could it be the ground equipment? Wouldn’t be the first time the localizer or glide slope goes haywire when a bus drives past the ground equipment.

[gulliBell]

Isn't there a magic black box you can plug the ILS receiver in to to check the proper electronic functioning of the system and indications? If they check out OK then the pitch servo theory comes next (assuming it's an aircraft type that has a dedicated pitch servo). Assuming the guys flying it know the magic number TQ reduction at top of drop that gets you on a pretty good GP all the way down without touching anything. Certainly in the S76 with SPZ7600, if you decouple the collective mode, the proper manual power reduction at the top of drop should have you on the GP all the way down without having to touch anything....maybe just a click or 2 of pitch trim every now and then.

[karote]

Thanks for the reply, yes we have tried on 4-5 different airfields, so thus we have ruled out the issue with ground equipment. It is this single airframe doing this. We have a second R/C but it is a 4-axis version and it flies nicely the ILS. 4-axis has the collective control and also accelerometers, so it is more advanced and thus not so good to compare with

[karote]

Hi Sir Korsky,

Thanks for the reply, yes we actually changed pitch rotary actuator as it had some extra friction causing it not to work smoothly. However, this did not solve this problem.
Has there been any problems with the jackscrew or the position sensors?

[karote]

Isn't there a magic black box you can plug the ILS receiver in to to check the proper electronic functioning of the system and indications? If they check out OK then the pitch servo theory comes next (assuming it's an aircraft type that has a dedicated pitch servo). Assuming the guys flying it know the magic number TQ reduction at top of drop that gets you on a pretty good GP all the way down without touching anything. Certainly in the S76 with SPZ7600, if you decouple the collective mode, the proper manual power reduction at the top of drop should have you on the GP all the way down without having to touch anything....maybe just a click or 2 of pitch trim every now and then.

Hi, thank you for your reply. We have check the functionality of the NAV receiver with the IFR4000 ramp generator and based on these measurements, the NAV radio is working and the EHSI shows deviations correctly. This is by the way B412.

"Assuming the guys flying it know the magic number TQ reduction at top of drop that gets you on a pretty good GP all the way down without touching anything" Could you please explain this a little bit more? I'm an avionic system engineer and not a pilot. I have tried to ask if the pilots are completely sure how to fly this 3-axis R/C and well, I believe them. But if there is something that I could still challenge them about , please let me know.

[havick]

What model 412? SP, HP, EP, EPI?

[gulliBell]

"Assuming the guys flying it know the magic number TQ reduction at top of drop that gets you on a pretty good GP all the way down...
For example, on capturing the GP, a TQ reduction of 7% at 120 kts will keep you on the GP without having to do much else....I forget what the power reduction is for the B412. But, by the sound of it, if the AP is making pitch changes, and the pilot hasn't been making any manual power changes, then something is not right and the mystery needs to be solved.

[[email protected]]

Are both the pitch channels behaving well - do you have the facility to monitor each channel's demand signal?

Can you fly a coupled ILS with just one pitch channel engaged? If so doing this with each AP deselected in turn might help you diagnose if it is an AP computer or servo problem.

[Salusa]

Are there not AP test Level 1 and 2 checks you can do on the ground? Either with engines running or hydraulics hooked up?

Error codes are usually generated and can at least point in right direction for troubleshooting.

Will usually display as 1Exx 2Exx for respective system.

The xx number will narrow it down though you will need Honeywell manual to refer. I.e, Bell MM not much help.

We had similar issue with 4 axis system on ILS GA 412EP. Turned out to be a accelerometer. Not saying that's your issue but run the BIT level 1 and 2 checks and see what codes you get.

I am assuming 3 axis has the same BIT modes.

Good luck.

[Encyclo]

As the great god of the SPZ system at Honeywell used to say:"friction, friction, friction"

You need to make sure all frictions are per the maintenance manual, not only flight controls but also hydraulic servo input levers. These are very precise values to check/set if you want the whole system to work.

Often overlooked are the hydraulic servo input levers, friction needs to be very low (refer to MM) and is often too high on new/overhauled servos due to the use of "D' rings in lieu of "O" rings to prevent leaks. Unfortunately, this is not adjustable and may require installing a different servo.

Fly Safe, Always :ok:

[karote]

Hi Havick,

It's EP.