Above 300 misbehave !!!

[tenten]

Avionics/radio & radar guys, have you experience this defect before?

C-130H, equipped with radio low range altimeter AL-101 (interfaces with GPWS, FCS-105, and TCAS)

Whenever the “CAN DO†hits the 300 AGL the “OFF FLAG†of Radio Alt. appears accompanied with failure of the other related system mentioned above.

Yet, unfortunately I am out of proper test equipment.

Voltage and continuity checks, done. Doesn’t show any abnormality.

Radio Alt. R/T changed.

Radio alt. indicator changed.

Still the bird behaving the same, above 300 feet flag “ON†below 300 feet back to normal operation.

Can I go for the antennas, and which one?

Any suggestions to this problem will be appreciated, since the next will be the 3rd FCF:confused:.

[larry myers]

OK Tiny, work your magic and help this young trooper out.

[tinyclark]

I've never worked this RAD ALT or the FCS-105. I don't know what the "can do" is.

I am assuming that there is a setting on the Altimeter Indicator, so that if you go below it, a flag pops up?? (MIN ALT?)

Is that what you are referring to, or are you saying that once the altitude is above 300', it shuts off and then shuts everything else off? Or if it goes above 300', you get TCAS errors, GCAS squawking ALT. The A/P shouldn't even be engaged below 300' should it?

I am just a bit confused at this point, since I've never worked on newer airplanes.

I would swap the transmit antenna first (left one). Have you checked the RF connectors, especially on the antenna ends, for corrosion?

[NATOPS1]

Tiny,

Sounds like the "can do" is the VALI (varible altitude limit indicator) for selecting the minimum alt for flight/ approach...anything above this and you "can do" it....?

With that said if it works below 300' then it sounds like the system is good just not enough "power" getting out.

As sugested check all you RF connectors (look for connectors that "spin")

If you have a similar aircraft and a TDR (time domain reflectometer) and a few "RF Adapters" hook into you good aircrafts RadAlt RF cables TX and RX and run a strip.

You will see the length of the ant cable and the type of termination (ant) open or short.

You can then compare your bad aircraft RF lines and see if they are different... if it is shorted you will get "less" power out and no Alt info above a certian height (300' maybe)

[tenten]

Teny, NATOPS1, thanks for the response and the advice which help me on the last corrective action.

Larry myers, thanks for pushing the “TOP†guys to react.

Apologize been late (Time dif. 8hrs ahead, weekend Friday & Saturday on my universe).

The “CAN DO†& “THE BIRD†simply I was referring to the airplane the “mighty Hercules†not the component nor the system, so apologize for that, I shouldn’t use those terms on technical issues.

For clarification of the defect, during the last FCF I was inboard with the flying crew, before rolling to T/off all the functional test of radio altimeter and the other related system (TCAS, GPWS) was as advertised… “OKâ€.

At initial climb the Radio Altimeter was reading smoothly, once the aircraft reached approximately the 300 feet AGL the warning flag of the Radio Altimeter comes in view followed by illumination of GPWS inoperative light, followed by “TCAS failed†on the MFD indicator, this condition remain during the climb and the cruise at 10,000 feet. During the landing phase (it was ILS Coupled Approach) the captain advised that some of the inputs such as (raising runway, decision height, GS deviation signal and capture point) will or might not be available or reliable spicily during the last phase of ILS landing due to the malfunction of the Radio Altimeter. Anyhow at approximately the 300 feet from touchdown the Radio Altimeter becomes a life, the FLAG disappeared and all the previous warning ended. The same scenario repeated twice.

Last finding and action taken is:-

1- One of the RF connectors points on the R/T mount receptacle (the one receiving the input from receiver antenna cable) found deteriorated (50% of contact wall found missing). Mount changed with new one.

2- Transmitter antenna changed with new one.

3- Due to operation requirement aircraft released today (Out Mission) without FCF, I am expecting a call from flying crew of the result, and once I get it, sure it will be posted.

As for general knowledge, of the AL-101 Low Range Altimeter, the normal system operation is to provide accurate terrain clearance between 0 and 2,500 feet, the indicator equipped with test switch, MDA set knob, minimum decision altitude light, inoperative red flag, and a mask at the end of the indicator scale.

The Radio Altimeter has three basic mode of operation:

Mode 1- Aircraft below 2,500 feet, normal return signal and no malfunctions detected. The flag is out of view and the indicator indicates the proper altitude.

Mode 2- Aircraft above 2,500 feet, and signal returns are normal. The flag is out of view and the indicator pointer is behind the pointer mask.

Mode 3- System functional test (manual) the flag is in view and the pointer indicates 100 feet.

As for system interface with other systems the radio altimeter interfaces with:

1-The FCS-105:- Inputs from Radio Altimeter used to desensitize the Glide Slope, controls the runway raising symbol (the raising up of the symbol starts from 200 feet to touchdown during ILS landing mode) and the decision height (DH) light on the ADI during approach to landing. Therefore it is not affecting the whole FCS-105 systems, auto-pilot and other mode of the system can be used during the malfunction of the Radio Altimeter.

2-GPWS:-The R/T provides altitude and flag alarm to the GPWS system, while the radio alt. indicator provides MDA to the GPWS, therefore almost the whole GPWS system depends on Radio Altimeter, in which GPWS inoperative warning light will come ON if Radio Altimeter malfunction.

3-TCAS:-Altitude received from Radio Altimeter is used by TCAS to determine distance to the surface (In order to inhibit or enable some modes of operation in relations to distance from ground level.

I hope this lengthy shallow information guides you to help me.

Thanks again for all of you.

[tenten]

Good day, I have got the feedback.

Defect improved a bit, but still acting and not accepted.

Now -according to flying crew report -defect shows as only momentarily cycling of the Radio Altimeter Indicator from time to time, the cycling starts in the altitude of approximately 500 feet AGL to 10,000 feet; above the 10,000 feet the indicator will stop cycling and behave as normal.

NO steady warning FLAG reported; and NO effect to other systems during the cycling.

I am still out of test kit; my intention upon aircraft arrival is to focus on both RF cables; and to change the receiver antenna.

The possibility of other system interference(GPWS,TCAS,FCF-105) causing the radio altimeter to fail is also knocking my head, but…......

Any other idea or technique to spot the defect will be appreciated.

[tinyclark]

Yea, sounds like it is just breaking lock to me. I'd really check those antenna connectors carefully. There may be corrosion on the center pin connection that you won't see unless you take them apart, especially if it was soldered. The newer stye connectors are crimp type pins for a reason.

I can't see any of the associated equipment doing it, but I have been wrong about 3 or 4 times before.

[tenten]

Thanks Tiny,

Connectors will be checked carefully, I am also request “TDRâ€/“VSWR†kit, they promised to provide.

I will post the finding.

[tenten]

As I promised to post the finding:

“The Good news†is: Aircraft now -out base- and is accomplished more than 5 sorties with no defect reported of Radio Altimeter.

“The Bad†is the cause of defect which is slightly “hazyâ€; due to been “pushed†to change two components at the same time.

-The R/T (which had been changed before 2 times with no effect).

-And the Receiver Antenna.

Why changing the R/T for the third time …!? What about the other two R/Ts .....?!! in which one of them is taken from a serviceable aircraft and fail to clear the defect.

I have no answer, but certainly “Radio Shop†and some “avionic guys†having the answer…..

I surely believe that the Receiver Antenna is the “culpritâ€

Anyhow……, Thanks to all for the help.

[tinyclark]

Yea, sounds like it was probably the receiver antenna, especially with known good R/Ts from other aircraft installed.