Avionics News April 2012 - 19

News & regulatory Updates
rTCA/EUrOCAE DO-178C, Software Considerations in Airborne Systems and Equipment Certification
Recently, RTCA issued the latest amendment and revision to DO-178. The revised document provides

EuROpE

recommendations for the production of software for airborne systems and equipment that performs its intended function with a level of confidence in safety that complies with airworthiness requirements. Compliance with the objectives of DO-178C is the primary means of obtaining approval of software used in civil aviation products. The new certification standard will be implemented on new projects and TSO/ETSO equipment once the standards are being updated or voluntarily applied. The related EUROCAE ED-12C will be issued soon.

FREQUENTLY ASKED QUESTIONS
Transponder Power Requirements
The following information is from various Federal Aviation Regulations and Technical Standard Orders. While we do not directly endorse one associate member over another, the following question was posed from one AEA member to Trig Avionics, and the answer submitted by Andy Campbell, field support engineer at Trig Avionics, is so thorough and compelling that it warrants publication in its entirety for the overall benefit of the membership.

QUESTiON: Where is the statement that “a Class 2 transponder may only be used for aircraft under 175 knots and 15,000 feet altitude” underpinned in regulation? I have never heard of this restriction before. Is this a European rule? I do not believe this rule is embraced in North America. For many years, I have sold KT-76A and KT-78A transponders, and this restriction has never been apparent. Interestingly, the integral Mode C encoder in the TT-21 is rated to 35,000 feet. Please confirm whether or not the TT-21 is really restricted to these low and slow aircraft. ANSWEr: The power requirements in North America are specified in the TSO documents. Before you start reading the small essay below, the KT-76A is a Class 1A transponder, and therefore, could be used above or below 15,000 feet/175 knots, which is probably why the restriction hasn’t been apparent before.
Transponders To find the statement for the power requirements of transponders, follow the chain down: Federal Aviation Regulation 91.215 states what airspace requires a transponder and that the transponder should be certified to TSO-C74 or TSO-C112.

The latest revision of TSO-C74, which is TSO-C74c, states power requirements for aircraft performance. For aircraft above 15,000 feet, a Class 1A transponder is required as a minimum, and below 15,000 feet, a Class 1B transponder is required. The latest revision of TSO-C112, which is TSO-C112c, states that the power requirements must be in accordance with the minimum operational standards for Mode S transponders in RTCA/DO181D. Our TT21 and TT22 transponders are certified to TSOC112c, and therefore, the DO-181D document applies. DO-181D paragraph 1.4.5 states: Two minimum peak output power levels are supported by this version of these MOPS, 125 watts (21.0 dBW), which is designated as Class 1 equipment, and 70 watts (18.5 dBW), which is designated as Class 2 equipment. For more information on output power levels, see §2.2.3.2. Class 1 equipment is intended for use in aircraft that operate at altitudes above 15,000 feet, or have a maximum cruising true airspeed in excess of 175 knots (324 km/h). Class 2 equipment may be used in aircraft that operate at altitudes not exceeding 15,000 feet, and have a maximum cruising true airspeed not exceeding 175 knots (324 km/h). There also is an amendment to TSO-C112c about the labeling of transponders, which changes the wording of transponder classes. The old Class 1A is now labeled as Level 1, Class 1. The old Class 1B is now labeled as Level 1, Class 2. The old Class 2A is now labeled as Level 2, Class 1. The old Class 2B is now labeled as Level 2, Class 2. The Level here refers to the communication capabilities of the transponder, and the Class still refers to the minimum output power. Our transponders are all Level 2 with our TT31 and TT22 transponders being Class 1 and the TT21 a Class 2 transmitter. This may be more information than you were hoping for,
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Avionics News April 2012

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