AFS-11-205-E 

 

May 24, 2011 

 Exemption No. 10269 

 Regulatory Docket No. FAA-2011-0188 

Mr. William M. Cavitt 

Vice President, Engineering, Performance and Quality Assurance 

American Airlines, Inc. 

3900 N. Mingo Road, MD 183 

Tulsa, OK 74116 

Dear Mr. Cavitt: 

This letter is to inform you that we have granted your request for exemption. It relays our 

decision, explains its basis, and gives you the conditions and limitations of the exemption, 

including the date it ends. 

The Basis for Our Decision: 

By letter dated February 24, 2011, you petitioned the Federal Aviation Administration (FAA) 

on behalf of American Airlines, Inc. (American), for an exemption from § 121.344(f) and 

part 121, Appendix M, of Title 14, Code of Federal Regulations (14 CFR). This exemption, if 

granted, would allow American the continued operations of the following American B737-

823 series aircraft for 18 months from the date of this letter without complying with 14 

CFR § 121.344(f) and part 121, Appendix M (footnote 18), the accuracy requirement for pitch 

control surface position and rudder control input force. 

The FAA has determined that good cause exists for waiving the requirement for 

Federal Register publication because the exemption, if granted, would not set a precedent and 

any delay in acting on this petition would be detrimental to American. 

The FAA has issued a grant of exemption to American Airlines in circumstances similar in all 

material respects to those presented in this petition. In Grant of Exemption No. 10173A 

(copy enclosed), the FAA granted relief from recording the parameters listed under 

14 CFR § 121.344(f) and part 121, Appendix M (footnote 18), because American minimized 

the impact of the loss of the flight recorder data and showed the loss of the data did not affect 

the safety of the aircraft. 
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Having reviewed your reasons for requesting this exemption, I find that— 

 they do not differ materially from those presented by American in the enclosed grant of 

exemption; 

 the reasons stated by the FAA for granting the enclosed exemption also apply to the 

situation you present; and 

 a grant of exemption is in the public interest. 

Flight data recorders (FDR) are required by regulation, because they are recognized as an 

essential tool for investigating accidents and incidents. Although a digital flight data recorder 

(DFDR) does not affect flightcrew procedures, it is critical for later analysis of crew 

performance and the procedures used during an accident or incident. 

The petitioner states that a Boeing 737 NG airplane elevator indication anomaly was 

discovered during ground operations that consequently led to the discovery of a flight data 

recorder system (FDRS) anomaly. Further investigation uncovered an identical anomaly 

associated with the recording of the rudder control input force. 

On October 19, 2010, a problem was detected on a Boeing production 737 NG airplane. The 

display of the left elevator position on the center lower display of the common display system 

(CDS) was lagging when compared to the right elevator position on the same display. The 

CDS retrieves surface position data from the digital flight data acquisition unit (DFDAU) for 

flightcrew display. The petitioner notes that the display of elevator position is not required to 

meet certification regulations and only supports an optional ground maintenance operation to 

aid in airplane dispatch. The display of the elevator positions (along with other primary flight 

control positions) on the CDS is a customer option. The petitioner’s aircraft are not equipped 

with this option. When the elevator indication problem was reported, the FDR was 

downloaded, and the data was analyzed to determine if the problem seen on the CDS was also 

present in the FDR elevator position data. This analysis also detected an identical problem 

with the rudder control input force recording. The FDR data analysis showed that, at times, 

one of the eight samples of data, in a given second, could be stale (repeating a previous 

sample) and is visible if there is a rapid movement of the elevator position or rudder controls. 

The anomalous data are usually present in each parameter only once every 5 seconds (i.e., 

once every 5 seconds there are seven valid samples and one invalid sample in a 1-second 

acquisition period, or subframe). The FDRS anomaly does not exist in the right elevator 

position data or the other flight control input force data. 

The root cause of this anomaly was found to be the result of a DFDAU software acquisition 

timing error. Periodically, in a given parameter acquisition cycle, the DFDAU is requesting 

the acquisition of more parameter data than it has time to process in that acquisition cycle. 

Because the parameter acquisition does not have time to complete the cycle, a previous valid 

sample is used in the recording rather than the desired current sample. 
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The petitioner states that Boeing had “verbally notified the FAA and is in the process of filing 

a report of noncompliance with part 25 airworthiness regulations with the FAA stating that 

the anomaly does not represent a safety issue for continued operation.” The petitioner also 

states that Boeing has a pending contract with Teledyne, the manufacturer of the DFDAU. 

The plan is to certify new DFDAU software by mid-August 2011. The date of certification is 

contingent on what is found during testing that may cause the schedule to change. Boeing is 

actively working to accelerate the certification of corrected software. Teledyne has informed 

Boeing that this anomaly can be fixed with a minor software revision resulting in new 

software part numbers. The DFDAU software is field loadable, which minimizes the 

workload and schedule impact of accomplishing this change. 

In support of allowing these aircraft to temporarily operate under this exemption, the 

petitioner notes that since the FDRS is recording the left elevator position and rudder control 

input force accurately at seven samples per second, there is not an adverse affect on the 

normal or irregular operation of the aircraft. The petitioner states that this anomaly can be 

addressed in ground processing of the data by eliminating the first sample, of eight, in each 

left elevator position acquisition cycle. 

The FAA’s Decision: 

In consideration of the foregoing, I find that a grant of exemption is in the public interest. 

Therefore, pursuant to the authority contained in 49 U.S.C. §§ 40113 and 44701, delegated to 

me by the Administrator, American Airlines, Inc., is granted an exemption from 

14 CFR § 121.344(f) to the extent necessary to allow American to temporarily operate newly 

delivered 737 NG aircraft, subject to the conditions and limitations listed below. 

Conditions and Limitations: 

1. This exemption only applies to the following aircraft. 

Make Model Serial# Line# Registration# 

Boeing 737-823 40762 3634 N867NN 

Boeing 737-823 40763 3668 N868NN 

2. This exemption covers only the recordation of the left elevator position, as part of 

the pitch control surface position parameter 15, and rudder control input force, as 

part of the flight control input force parameter 88, listed in 14 CFR part 121, 

Appendix M. 

3. A copy of this exemption must be retained with the operator’s DFDR correlation 

documentation as specified in 14 CFR 121.344(j) until the airplanes identified in 

this exemption have incorporated the DFDAU system modification. 

4. In the event of an accident or incident after which the DFDR data must be sent to 

the National Transportation Safety Board, American must provide a copy of this 
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exemption and instructions on how to reprogram the DFDR ground read-out tool 

to eliminate the anomaly. 

5. Following receipt of the modification service bulletins, American must provide, 

to its principal avionics inspector, a schedule for the completion of the new 

installation. 

This exemption terminates on the sooner of the installation of the DFDAU software update 

correcting the problem described in the petition, or 18 months after the date of this letter,

unless sooner superseded or rescinded. 

Sincerely, 

/s/ 

Raymond Towles 

Acting Director, Flight Standards 

 Service 

Enclosure