58966 Federal Register / Vol. 70, No. 194 / Friday, October 7, 2005 / Proposed Rules 

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. 

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. 

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DATES: Notice No. 96–5 is withdrawn 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. 

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 

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,