[Federal Register Volume 70, Number 194 (Friday, October 7, 2005)]
[Proposed Rules]
[Pages 58966-58967]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 05-20181]



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Part IX





Department of Transportation





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Federal Aviation Administration



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14 CFR Parts 121 and 135



Mode S Transponder Requirements in the National Airspace System; 
Proposed Rules

  Federal Register / Vol. 70, No. 194 / Friday, October 7, 2005 / 
Proposed Rules  

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 121 and 135

[Docket No. 28586; Notice No. 96-5]
RIN 2120-AE81


Mode S Transponder Requirements in the National Airspace System

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking; withdrawal.

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SUMMARY: On May 23, 1996, the Federal Aviation Administration (FAA) 
proposed to rescind the requirement to install a Mode S transponder on 
all aircraft operating under parts 135 and those aircraft operating 
under part 121 of Title 14 of the Code of Federal Regulations that are 
not equipped with Traffic Collision and Alert System (TCAS) II. The FAA 
has, however, reassessed the need for retaining this requirement and 
now withdraws that notice.

DATES: Notice No. 96-5 is withdrawn October 7, 2005.

FOR FURTHER INFORMATION CONTACT: Mr. K. Douglas Davis, Avionic Systems 
Branch, Aircraft Certification Division, Federal Aviation 
Administration, 800 Independence Avenue SW., Washington, DC 20591; 
telephone (202) 385-4636.

SUPPLEMENTARY INFORMATION: You can get an electronic copy of this 
document by going to our Web page at http://www.faa.gov/avr/arm/nprm.htm or by using the search feature of the Federal Register's Web 
page at http://www.access.gpo.gov/su_docs.
    You can get a printed copy by sending a request to the Federal 
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence 
Avenue, SW., Washington, DC 20591, or by calling (202) 267-9680. Please 
identify the docket number of this final rule.

Background

    The FAA published a final rule on February 3, 1987 (52 FR 3380), 
requiring the installation of Mode S transponders on U.S. registered 
aircraft. The rule provided that any transponder newly installed before 
January 1, 1992, in aircraft used for operations under parts 121 and 
135 of the Federal Aviation Regulations could be a Mode A or Mode C 
transponder if the transponder was manufactured prior to January 1, 
1990. After January 1, 1992, only Mode S transponders could be newly 
installed in U.S.-registered civil aircraft operated under Parts 121 
and 135. The rule also provided that Mode S transponders were required 
for Part 91 operations in specified designated airspace.
    The FAA delayed the rule's effective date for 6 months until July 
1992 due to manufacturing difficulties of Mode S transponders. 
Additionally, the Air Traffic Issues Group of the Aviation Rulemaking 
Advisory Committee recommended to the FAA that the Mode S requirement 
for Part 91 was premature and deserved further study. In light of this 
recommendation, we reviewed the issue and concluded that the 
requirement was not necessary for Part 91 operators. Specifically, we 
determined that ``While areas of high density aircraft activity might 
benefit from the improved target and altitude integrity of the Mode S 
system, many portions of the airspace over the country might not 
require a homogenous Mode S environment for several years.'' 
Subsequently, we rescinded this equipment requirement for Part 91 
operations in 1992 (57 FR 34614; August 5, 1992).
    On May 23, 1996, the FAA proposed rescinding the Mode S requirement 
for Part 135 operators and those Part 121 operators not required to 
have TCAS II (61 FR 26036). The FAA has reassessed the need for Mode S 
transponder equipage for these operators in today's transportation 
system.

FAA Assessment and Review

    In the May 1996 NPRM (Notice 96-5), we articulated several reasons 
for proposing to rescind the Mode S transponder requirement because: 
(1) The FAA's revised strategy of multiple air-ground data links 
managed through Aeronautical Telecommunications Network would remove 
the requirements for Mode S transponders;\1\ (2) operational experience 
with Mode S ground sensors has shown that most surveillance 
enhancements can be achieved by the Mode S ground sensors with the 
present mixed population of airborne transponders; and (3) the use of 
Mode S transponders for aircraft, other than those required to have 
TCAS II, does not offer, nor is it expected to offer, any significant 
safety advantage in the current or future environment.
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    \1\ In our review, we determined that this specific rationale 
was incorrect. Data link capability is not required and moreover, 
the use of the Mode S transponder only will not provide that data 
link services.
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    Since Notice 96-5 was published, the National Airspace System (NAS) 
has experienced significant changes that now necessitate retaining the 
Mode S transponder requirement. The basis for this requirement, 
however, has changed due to advances in the technological use of Mode S 
systems, and the continued growth of aviation traffic. Mode S 
technology has the largest ground and airborne infrastructure in place. 
Both industry and the FAA have invested in this infrastructure, and as 
discussed below, new technology and new safety enhancement programs 
utilize Mode S. Thus, we conclude that Mode S transponders are 
necessary for part 135 and part 121 operators (those not required to 
have TCAS II) and provide benefits beyond those considered in 1996.
    Mode S avionics now are available for all types of aircraft from 
general aviation to heavy commercial aircraft. Virtually all commercial 
aircraft manufactured within the last 10 years have been equipped with 
Mode S transponders. In addition, over the last decade, the FAA has 
modernized many ground-based systems with considerable investment 
placed on Mode S capability--enhancing the operational picture 
available to aircrews and air traffic control.
    The FAA has developed and implemented safety enhancements that 
either did not exist at the time that Notice 96-5 was published or that 
perform better with Mode S transponders than with Mode A or Mode C 
transponders. Airport Surface Detection 3 (ASDE-3), ASDE-X and Airport 
Movement Area Safety Systems (AMASS) are the core of supporting 
surveillance services for airport surfaces at approximately 60 of the 
busiest airports around the United States. These systems were all 
implemented after 1996.
    ASDE-3 was the first surface detection system to become 
operational, and when used with the AMASS system, it provides a safety 
alerting service that warns the controllers of possible runway 
incursions.
    ASDE-X is the next generation of surface detection systems and adds 
multilateration transponder based surveillance, both ground based and 
airborne, that is used with ASDE-3. The multilateration capability of 
ASDE-X allows the system to provide much more reliable overall 
surveillance than the ASDE-3 AMASS system. Increasing use of Mode A or 
Mode C transponders in lieu of Mode S transponders will weaken the 
surveillance capability of an ASDE multilateration system (as discussed 
below) and increase the risk of false or missed alerts.
    The multilateration component of these systems uses fixed wide beam 
antennas when it interacts with transponders, and is thus more 
susceptible to synchronous garble and

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Frequent Replies to Unwanted Interrogation Transmissions (FRUIT). FRUIT 
occurs when one system detects replies that match another system's 
interrogation. When FRUIT replies are received at the same time as the 
reply that actually matches the system's interrogation, these replies 
will garble the matching reply. A combination of the timing of the 
interrogation and its matching reply allows the development of the 
surveillance data in determining the location and altitude of the 
aircraft. The garbling of these transmissions corrupts the surveillance 
data.
    Synchronous garble occurs when a ground sensor or a TCAS (airborne) 
interrogate Mode A or Mode C aircraft and receive replies from more 
than one aircraft at the same time. Again, the replies garble each 
other and their corresponding data may be lost. Reducing the number of 
Mode A and Mode C transponders will reduce the frequency of this garble 
and corruption of the data. Mode S systems use addressed interrogations 
where only one Mode S transponder replies at a time.
    We also have found that Mode S transponders perform better than 
Mode C or Mode A transponders with respect to the Traffic Information 
Service (TIS). TIS avionics is optional and provides pilots with 
information on nearby traffic. It operates in Mode S radar systems. The 
Mode S radar system receives requests from TIS avionics through a Mode 
S transponder onboard an aircraft. The Mode S radar system processes 
the request from the TIS avionics and transmits basic traffic 
information to the requesting aircraft for processing and display to 
the pilot. This information includes distance and bearing to the 
traffic, for up to eight aircraft. This information also allows the 
pilot to look out the window and find the aircraft more effectively.
    ADS-B is a system where aircraft automatically transmit 
surveillance data derived from navigation position data, e.g., GPS. 
ADS-B, when using the 1090 extended squitter application, uses Mode S 
transponder transmissions. Just as FRUIT garbles regular transponder 
reply transmissions, FRUIT will garble these transmissions. Excessive 
FRUIT will lower the capacity of ADS-B and reduce its usefulness, and 
any related safety and efficiency benefits.
    TCAS is a system installed in aircraft to help avoid midair 
collisions. Mode A and Mode C transponders can affect TCAS in two ways. 
First, the TCAS may experience FRUIT, synchronous garble or transponder 
dead time, which results in reduced ability to detect and track 
aircraft and provide its collision service effectively. Secondly, the 
presence of Mode A and Mode C transponders may limit the TCAS's range 
of operation. The fewer Mode A and Mode C transponders operating around 
TCAS equipped aircraft, the greater the range the TCAS may operate, 
which maximizes its safety benefit. Decreasing the numbers of Mode A/C 
transponders decreases the risk of missed alerts and false alerts.
    Mode S provides benefits over Mode C or Mode A transponders during 
interrogation and transponder dead time. ``Dead time'' is when the 
transponder is busy. The transponder is kept busy when processing 
interrogations and preparing/transmitting a reply. The transponder also 
is kept busy with processing interrogations with an indication to 
suppress and not transmit an unnecessary reply. In systems such as 
ASDE-X, FRUIT replies from Mode A and/or Mode C aircraft can be 
received at the same time as the Mode S reply matching the 
interrogation. These garbling FRUIT replies can cause the loss of the 
Mode S reply. While the Mode S protocols prevent data corruption, the 
ASDE-X system will re-interrogate the Mode S transponder again in an 
effort to get the needed reply. This increases the transponder's dead 
time through over-interrogation. The Mode S interrogations also include 
an indication to Mode A and/or Mode C transponders to suppress and not 
reply. This protects against synchronous garble. Consequently, while 
the Mode S interrogations are intended primarily for a Mode S 
transponder, the Mode A and/or Mode C transponders anywhere near the 
path from the Mode S transponder to the radar or TCAS will see the 
suppression indication. Again, as with over interrogation, these 
transponders are also kept more busy than necessary, which increases 
their dead time as well.
    Overall, the selective interrogation and the superior resolution 
ability of Mode S eliminates synchronous garble; resolves the effects 
of over interrogation; simplifies aircraft identification; and allows 
Mode S integration with new technologies designed to improve efficiency 
in the NAS.
    Since the NPRM, the European Union and similar global coalitions 
have implemented equipage mandates, including Mode S, to operate in 
their airspace. Consequently, the FAA is working to synchronize and 
bridge equipage gaps to ensure that the United States' aviation 
economic interests around the world are maintained.
    Published concurrently with this notice, is a separate notice 
seeking public comment on a proposed date for the equipage of Mode S 
transponders for aircraft that have been operating under FAA issued 
exemptions from this requirement.

Withdrawal of the NPRM

    Since Mode S transponders can provide improved safety and 
efficiency in a more densely populated NAS, the FAA has concluded that 
the Mode S requirement for Part 135 and certain Part 121 operators 
remains valid. Therefore, the FAA withdraws NPRM 96-5.

    Issued in Washington, DC on September 28, 2005.
James J. Ballough,
Director, Flight Standards Service.
[FR Doc. 05-20181 Filed 10-6-05; 8:45 am]
BILLING CODE 4910-13-P