Exemption No. 10173 

UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

WASHINGTON, DC 20591 

 

In the matter of the petition of 

 

AMERICAN AIRLINES, INC. Regulatory Docket No. FAA-2010-1178

 

for an exemption from § 121.344 and 

Part 121 Appendix M of Title 14, 

Code of Federal Regulations 

 

GRANT OF EXEMPTION 

 By letter dated November 23, 2010, Mr. William Cavitt, Vice President, Engineering, 

Performance, and Quality Assurance American Airlines, Inc. (American), 3900 N Mingo 

Road MD 183, Tulsa, OK 74116 petitioned the Federal Aviation Administration (FAA) on 

behalf of 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 Airlines B737-823 series 

aircraft for 18 months from December 6, 2010 without complying with 14 CFR § 121.344(f) 

and part 121 Appendix M (footnote 18) parameter accuracy for pitch control surface position. 

The petitioner requests relief from the following regulations:

Section 121.344(f) prescribes that for all turbine-engine-powered transport category airplanes 

manufactured after August 19, 2002, the parameters listed in paragraphs (a)(1) through 

(a)(88) of this section must be recorded within the ranges, accuracies, resolutions, and 

recording intervals specified in appendix M to this part. 

Appendix M, footnote 18 prescribes that for all aircraft manufactured on or after December 6, 

2010, the seconds per sampling interval is 0.125. Each input must be recorded at this rate. 

Alternately sampling inputs (interleaving) to meet this sampling interval is prohibited. This 

footnote is applicable to parameters 12a, 12b, 13a, 13b, 14a, 14b, 15, 16 17, and 88. The 

petitioner specifically requests relief for parameter 15, pitch control surface(s) position, left 

elevator position. 

AFS-11-059-E 
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The petitioner supports its request with the following information:

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

discovered during ground operations which consequently led to the discovery of a Flight Data 

Recorder System (FDRS) anomaly. 

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 flight crew 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 Flight 

Data Recorder (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. The FDR data analysis 

showed that at times, 1 of the 8 samples of left elevator position data, in a given second, could 

be stale (repeating the previous sample) and is visible if there is a rapid movement of the 

elevator position. The anomalous left elevator position data is usually present only once every 

5 seconds (i.e. once every 5 seconds there are only 7 valid samples and one invalid sample in 

a one second acquisition period, or sub frame). The FDRS anomaly does not exist in the right 

elevator position data. 

The root cause of this anomaly was found to be the result of a digital flight data acquisition 

unit (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, the previous valid sample is used in the recording rather than the 

desired current sample. 

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. 

After the contract is accepted, the plan is to certify new DFDAU software in approximately 5-

6 months. The date of certification is contingent on what is found during testing which 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 

alteration. 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 accurately at 7 

samples per second, there is not an adverse affect on the normal or irregular operation of the 
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aircraft. The petitioner states it is possible to deduce a corrected left elevator position through 

a comparison with the reading of the right elevator position or to linearly interpolate between 

the succeeding and preceding left elevator position samples to temporarily mitigate this 

anomaly. Also, the petitioner contends 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. 

Notice and Public Procedure: 

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's analysis is as follows:

Flight data recorders are required by regulation because they are recognized as an essential 

tool for investigating accidents and incidents. Although an FDRS does not affect flight crew 

procedures, it is critical for later analysis of crew performance and the procedures used during 

an accident or incident. 

The FAA agrees that although this anomaly results in the system not meeting the accuracy 

requirements of the regulations, the FDRS provides sufficient information to allow the 

airplane to operate in revenue service for the time period specified in this exemption. In the 

event of an accident or incident, investigators would have adequate information with the data 

recorded at 7 samples per second and with mitigation techniques available to approximate the 

movement of the left elevator. The FAA agrees that the right elevator position can be used to 

track the left elevator position information under normal operating conditions, but in the event 

of an elevator malfunction, this data may not be reliable or appropriate. 

Delaying the introduction of these airplanes into service could result in a negative effect on 

commercial air service and the integration of newer, safer, more fuel efficient airplanes in 

passenger service. Boeing is continuing its work with Teledyne to correct the DFDAU 

software and will provide the modified software to the petitioner to incorporate into its fleet 

as soon as it is available. The petitioner’s arguments for allowing continued operation of the 

airplanes with the software anomaly that minimally affects one parameter is acceptable. 

Based on these considerations, the FAA has determined that it is in the public interest to 

allow the introduction of the subject aircraft into service while the problem is being 

addressed. 
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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 while registered to and 

operated by American Airlines, Inc.: 

Make Model Serial# Line # Registration

# 

Boeing 737-823 30903 3481 N863NN 

Boeing 737-823 31111 3487 N864NN 

Boeing 737-823 29554 3493 N865NN 

Boeing 737-823 40584 3499 N866NN 

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

the pitch control surface position parameter 15 listed in 14 CFR part 121, 

Appendix M. 

3. A copy of this exemption must be retained with the operator’s flight recorder 

correlation documentation until all airplanes identified in this exemption have 

incorporated the system modification that corrects this anomaly. 

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

the FAA or the NTSB, American must provide: 

a. a copy of this exemption, and 

b. instructions on how to reprogram the DFDR ground read-out tool to 

eliminate the left elevator parameter anomaly. 

5. Following receipt of the modified DFDAU software, American must provide to 

its Principal Avionics Inspector (PAI) a schedule for the completion of the new 

software installation for each affected airplane. 

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 the exemption, 

unless sooner superseded or rescinded. 

Issued in Washington, DC, on December 9, 2010. 

/s/ 

Raymond Towles 

Acting Director, Flight Standards Service