Q&A: Collins Radio Altimeter
November 2009
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Schultz, Gerry - Avionics Tech Rep - Lincoln, NE (LNK). Aviation professional since 1965. Airframes: Hawker, King Air Instrumentation, Vertical and Directional Gyros, Airspeed Indicators, HSI, ADI, Encoders, Course Indicator, Rate Gyro, Directisyn, Vertisyn, RMI and Pitot Static Components and Systems Specialist. Direct: 402.479.4212 |
Question
Gerry,
I have a Collins Radio Altimeter model ALT50A in a Lear 25 that appears to be bad. On the ground it checks out OK, reading about correct alt and indicates 50 ft. with push-to-test. The pilot indicated it saturated around the time/altitude the gear went up.
I also have an ALT55B in another aircraft. The DOM talked to someone who told him we can’t put the 55B in the 50A tray. About the same time, I talked to Rockwell Collins and was told a 55B can go in a 50A tray, but not vice-versa. The power pins seem to be the issue. Collins said the 55B would automatically adapt, whereas the 50A could not. I'd rather be safe than sorry. Can you verify this either way?
In a related question, what is the signal output of the 55B?
The 50A I have:
- -20 to 500 ft: 20mV/Ft, -20 ft = 0V
- 500 to 2000 ft: 10.4V + 3mV/Ft
My indicator only goes to 2000 ft. What happens if the 55B is used, which goes to 2500 ft? Should I or do I need to also change the indicator?
Bill
DOM, Arizona
Answer
Bill,
Collins is correct. You can install a 55B in place of your 50A, but not vice-versa.
I talked with my tech about the signal output and the output voltages are the same on both receiver transmitters. As far as the ceiling, you are correct. The 50A goes to 2000 ft. and the 55B goes to 2500 ft. All that will happen is if the indicator goes above 2000 ft it will go out of view. It will not hurt anything.
Gerry Schultz, Avionics Tech Rep
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