ALT-55B 622-2855-001

Rockwell Collins - Radio Altimeter Transceiver
Part Number :

  • Lightweight, low-profile radio altimeter compatiable with Ground Proximity Warning Systems (GPWS)
  • Typically used with ALI-55/55A analog indicators or DRI-55 digital display indicator, and two 437X-1/1A/1B or ANT-50A/51A/52 antennas depending on aircraft configuration
  • Provides decision height selection from 0 to 2500 feet
  • Provides four shop-adjustable altitude trips (-001/011 unit)
  • Capable of dual system configuration
  • Built-in self-test effectively checks system excluding antennas
  • Version available with altitude error reduction where unit will indicate lower altitude in the event of unit failure thus ensuring safe terrain separation (-01X version - see table below)
  • No longer manufactured New - See ALT-1000 or ALT-4000 for alternative
FAA TSO: C87 RTCA: DO-138, DO-123
FCC Rules: Part 87 Environmental Category: DO-138/AG/A/JN/AAAEXXXXX
Temperature: -54 to 71C (operating) Service Altitude: Up to 55000 feet
Analog Accuracy: +-2 ft. or +-2% Transmitter Power: 350 mW nominal
Frequency Modulation (Transmit): 100 or 105 Hz (selectable) Load Capability: 1 millimho maximum
Time constant: 0.09 +-0.01 s System Accuracy: +-5 ft. or +-5% (0-500 ft)
Power Requirements: +22 to +34 Vdc. 30W (+28Vdc nominal) ARINC: 408
Part NumberRadio Altimeter Transceiver Description:
622-2855-001 -20 to 2500 ft. range with 4 trip shop adjustable trip outputs
622-2855-011 Same as -001 but with reduced altitude error product improvement mentioned above
622-2855-002 -20 to 2500 ft. range with 4 trip non-shop adjustable trip outputs
622-2855-012 Same as -002 but with reduced altitude error product improvement mentioned above


- Radio Altimeter Transceiver

NSN: 5841-01-272-2715

Price Condition Delivery

SEA Repair Capabilities: Yes

- Radio Altimeter Transceiver
Price Condition Delivery

SEA Repair Capabilities: Yes

- Radio Altimeter Transceiver

NSN: 5841-01-429-4799

Price Condition Delivery

SEA Repair Capabilities: Yes

- Radio Altimeter Transceiver
Price Condition Delivery

SEA Repair Capabilities: Yes

Click on a question below to see the answer. If you have a question about this model that is not answered below, please contact

The ALT-55B contains a circuit designated as AID (Aircraft Installation Delay). This circuit compensates for differences in cable lengths and the distance from the bottom of the aircraft to the ground when on the ground. This is selected by program strapping during the installation.

ALT-55B's with dash number -011 have Service Bulletin 6 installed. This modification accomplishes the following:

In the event of an AID failure, the indicated altitude will increase significantly, and this increase will be undetected in flight. After incorporation of this modification, an ADI strap failure will result in a decrease of indicated altitude, thus ensuring safe terrain separation. Incorporation of this Service Bulletin is for safety reasons and is optional.

Service Bulletin 5 is the same modification but does not require changing the dash number from -001 to -011.

Dirty antennas can sometimes cause poor or erratic operation of the radio altimeter system. Especially during winter months, some aircraft's nose wheel's throw dirt and slush onto the antenna. As a preventative measure to keep the antennas clean, a quality aircraft wax can be applied to the antenna.
Display lockup is a commonly encountered problem where the indicator locks up at a low indicated altitude between 5-50 feet. Some pilots will report that the radio altimeter will work normally at low altitudes but after climbing above a certain altitude the indicator locks up. Once the aircraft descends past the locked altitude, the radio altimeter works normally again.

In almost all cases, the cause of lockup is excessive signal leakage between the receive and transmit antennas. Signal leakage is commonly caused by the following situations:

- Antenna reflections hitting an object below the fuselage such as landing gear, skids, or searchlights.
- Poor ground plane between the antennas and the airframe, antennas not mounted on the same piece of metal, or antennas mounted on composite material
Sometimes a pilot may report that the radio altimeter may start indicating altitudes in the rad alt range (0-2500 ft.) at altitudes between 18000-20000 feet. This frequently occurs when flying over a reflective surface such as calm water. What occurs with second time around acquisition is that the receiver detects a return signal that was transmitted two pulses earlier not the transmitted signal it should be receiving. The time between transmitted pulses is equal to a distance in the 18000-20000 ft. range. Therefore, the receiver may process it as though it was signal it was actually trying to receive and then display the incorrect altitude.
Large needle fluctuations is a problem commonly seen when a helicopter is hovering over a soft, diffused surface such as grass. Needle jumps are usually less than 50 feet but can range from hundreds to a thousand feet. Once the helicopter moves forward, the radio altimeter should operate normally again. This needle jumping occurs because the radio altimeter is experiencing decreases in received signal strength. Over a soft, diffused surface, the signal reflection consists of hundreds of small weak reflections with different time and phase, directional qualities. These differences cause cancellations in the total return signal and cause the needle to jump.

To correct this needle jumping, some radio altimeters have an extended dB STC range. This extended range provides an additional receiver gain that can reduce the decreases in signal strength. However, the additional dB gain increase can subsequently increase the potential for display lockup caused by poor isolation between the antennas.
In civilian aviation, these two terms are used interchangeably and essentially mean the same. They both can be abbreviated as "Rad Alt" sometimes as well.

This system measures the between an aircraft and the ground directly below it. "Radar" or Radio Detection and Ranging is the principle by which the system operates. That is, a signal is transmitted towards the ground and then received back for processing. The time the signal takes to reflect back to the aircraft is timed and this is how the altitude is measured. The signal that is transmitted is a radiowave. Thus, this is perhaps this is the reason why some may use the term "Radio Altimeter" instead of "Radar Altimeter".
Southeast Aerospace exchanges are based on the return of an undamaged, economically repairable core unit with identical part number as the unit shipped to the customer. An "economically repairable" core is defined as one where the cost to repair/overhaul (or Repair Cap) does not exceed 80% of the original SV/OH exchange price billed. Should the Repair Cap exceed 80%, the customer will be billed the additional amount. In the event this amount exceeds the Outright Price for the unit, the customer would only be billed the difference between the Outright Price and SV/OH Exchange Price with the core returned as-is to the customer.

Here is an example of such a transaction:

$1000 Exchange Price charged to customer
$1000 x .8 = $800 Maximum Allowable core repair charge or Repair Cap

$2000 Cost to repair core unit
- $800 Less Core repair cap
$1200 Additional billing amount.

2200.00 Total Cost of transaction

Please Note:
SEA offers exchange on new items as well. However, repair cap as indicated above is still based on SV/OH exchange price.

Negotiating the exchange price of a unit only limits the allowable repair cap for the core unit. Southeast Aerospace's exchange transactions are based on the return of economically repairable core unit. Once the core is received and evaluated, the core repair cost incurred by SEA cannot exceed 80% of the original exchange price. That is, it cannot cost SEA more than 80% of the original OH/SV exchange price collected from the customer. Therefore, when and if an SEA exchange price is discounted, there is a risk that additional charges may be assessed once the core is returned and evaluated.

For more information, please refer to these other Exchange FAQs

The "stripline" in all avionics equipment is an assembly that operates in the microwave frequency range (usually.3 GHZ to 30 GHZ) in the RF assembly of the unit. It contains all stages that operate in that range, such as Local Oscillators, RF amplifiers, heterodyne mixers, transmission lines (called waveguides).

Avionics components such as DMEs, transponders, radio altimeters, and weather radars commonly include stripline assemblies.