Federal Register / Vol. 70, No. 194 / Friday, October 7, 2005 / Proposed Rules 58967 

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 reinterrogate 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 

DEPARTMENT OF TRANSPORTATION 

Federal Aviation Administration 

14 CFR Parts 121 and 135 

[Docket No. FAA–2005–22593] 

Mode S Transponder Requirements in 

the National Airspace System 

AGENCY: Federal Aviation 

Administration (FAA), DOT. 

ACTION: Notice of policy; request for 

comments. 

SUMMARY: This notice of policy 

announces the FAA’s policy concerning 

current exemptions from the Mode S 

transponder equipment requirements 

under Title 14 of the Code of Federal 

Regulations §§ 121.345(c) and 

135.143(c). Additionally, this notice of 

policy seeks comments from persons 

currently holding an exemption from 

the above regulations on a proposed 

date for which they must comply with 

the equipment requirements. 

DATES: Comments must be filed no later 

than November 7, 2005. 

ADDRESSES: You may send comments to 

Docket Number 22593 using any of the 

following methods: 

• DOT Docket Web site: Go to 

http://dms.dot.gov and follow the 

instructions for sending your comments 

electronically. 

• Government-wide rulemaking Web 

site: Go to http://www.regulations.gov 

and follow the instructions for sending 

your comments electronically. 

• Mail: Docket Management Facility; 

U.S. Department of Transportation, 400 

Seventh Street, SW., Nassif Building, 

Room PL–401, Washington, DC 20590–

001. 

• Fax: 1–202–493–2251. 

• Hand Delivery: Room PL–401 on 

the plaza level of the Nassif Building, 

58968 Federal Register / Vol. 70, No. 194 / Friday, October 7, 2005 / Proposed Rules 

400 Seventh Street, SW., Washington, 

DC, between 9 a.m. and 5 p.m., Monday 

through Friday, except Federal holidays. 

Privacy: We will post all comments 

we receive, without change, to http:// 

dms.dot.gov, including any personal 

information you provide. For more 

information, see the Privacy Act 

discussion in the SUPPLEMENTARY

INFORMATION section of this document. 

Docket: To read background 

documents or comments received, go to 

http://dms.dot.gov at any time or to 

Room PL–401 on the plaza level of the 

Nassif Building, 400 Seventh Street, 

SW., Washington, DC, between 9 a.m. 

and 5 p.m., Monday through Friday, 

except Federal holidays. 

FOR FURTHER INFORMATION CONTACT: Ida 

Klepper, Office of Rulemaking, Federal 

Aviation Administration, 800 

Independence Avenue, SW., 

Washington, DC 20591, (202) 267–9677, 

email: Ida.Klepper@faa.gov. 

SUPPLEMENTARY INFORMATION: 

Background 

Concurrent with the publication of 

this notice and in a separate part of this 

Federal Register publication, the FAA 

announces that it is withdrawing Notice 

No. 96–5, a Notice of Proposed 

Rulemaking to rescind the Mode S 

transponder requirements for aircraft 

operating under Title 14 of the Code of 

Federal Regulation (CFR) part 135 and 

aircraft operating under Part 121 that are 

not required to have Traffic Alert and 

Collision Avoidance Systems (TCAS) II. 

Since Notice 96–5 was published in 

May 1996, the FAA has granted a 

number of exemptions to the above 

referenced sections as the agency 

progressed toward removing this 

equipment requirement from all aircraft 

except those aircraft operated under Part 

121 that have TCAS II. As the agency 

has revised its long-term plan for Mode 

S and is retaining this equipment 

requirement, we want to provide a 

reasonable timeframe for the 

exemptions to terminate and for affected 

operators to comply with the applicable 

regulations. 

Proposed Termination Date of 

Exemptions 

We propose that all FAA authorized 

exemptions of 14 CFR 121.345(c) and 

135.143(c) terminate no later than 

March 1, 2007. Until that date, the FAA 

will review on a case-by-case basis 

requests to extend current exemptions 

to that date. The FAA, however, would 

not be granting any new exemptions 

after March 1, 2007. This proposed date 

would provide affected operators with 

approximately 18 months to install a 

Mode S transponder if necessary under 

§§ 121.345(c) and 135.143(c). 

Under paragraphs (c) of §§ 121.345 

and 135.143, after January 1, 1992, only 

Mode S transponders may be newly 

installed in U.S. registered civil aircraft. 

Under these regulations, the term 

‘‘installation’’ does not include: (1) 

Temporary installation of a Mode C 

transponder or substitute equipment as 

appropriate, during maintenance of the 

permanent equipment; (2) reinstallation 

of equipment after temporary removal 

for maintenance; or (3) for fleet 

operations, installation of equipment in 

a fleet aircraft after removal of the 

equipment for maintenance from 

another aircraft in the same operator’s 

fleet. 

Consequently, the FAA proposes that 

effective March 1, 2007, if a transponder 

needs to be permanently replaced it 

must be replaced with a Mode S 

transponder. This does not mean that 

effective March 1, 2007, operators are 

required to install Mode S transponders, 

if they have an operable and appropriate 

Mode C or Mode A transponder in the 

aircraft. The current regulation only 

requires the replacement of a Mode A or 

C transponder with Mode S when the 

existing transponder can no longer be 

repaired. In addition, the FAA notes 

that if you were (or are) issued an 

exemption before March 1, 2007, 

allowing you to install a Mode C 

transponder on your aircraft, you may 

use that transponder until it no longer 

can be repaired and must be replaced 

with a Mode S. Therefore, if you hold 

an exemption, any Mode A or C 

transponder that is installed on or 

before March 1, 2007, may remain in 

your aircraft and may continue to be 

repaired after March 1, 2007. When that 

Mode A or C transponder can no longer 

be repaired, it must be replaced with a 

Mode S transponder. After March 1, 

2007, you will not receive an exemption 

to allow the installation of a Mode A or 

C transponder to replace a Mode A or 

C transponder that cannot be repaired. 

The FAA invites all affected operators 

to comment on the proposed date of 

March 1, 2007, as the latest termination 

date for authorized exemptions from 

§§ 121.345(c) and 135.143(c). After that 

date, any transponder newly installed in 

U.S. registered aircraft must be Mode S. 

Issued in Washington, DC on September 

27, 2005. 

Anthony Fazio, 

Director of Rulemaking. 

[FR Doc. 05–20183 Filed 10–6–05; 8:45 am] 

BILLING CODE 4910–13–P