[Federal Register: October 7, 2005 (Volume 70, Number 194)]
[Proposed Rules]
[Page 58965-58967]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr07oc05-24]
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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
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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DATES: Notice No. 96-5 is withdrawn October 7, 2005.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]
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