AEROSPACE 

123 Garratt Blvd 

Downsview, Ontario 

Canada M3K 1Y5 

www.bombardier.com 

TEL 416-633-7310 

FAX 416-375-4525 

November 22nd, 2013 

File: 8-21-1 Rev. - 

ATTENTION: FAA, Regulations

 Republic Airlines & Horizon Air 

SUBJECT: DHC-8 Series 400 

 FAA 14 CFR Part 121.346 Flight Data Recorders: Filtered Data 

 - Request of three months exemption for compliance from FAA 

REFERENCE: Honeywell Quality Assurance CM CSB-104980 Rev 2, Nov 22nd, 2013 

To whom it may concern, 

The purpose of this letter is to support Horizon Air and Republic Airlines of requesting FAA for 

granting a three month exemption after April 21, 2014 for demonstrating compliance with FAA 

14 CFR Part 121.346 Flight Data Recorders: Filtered Data. 

BACKGROUND

DHC-8 Series 400 System Description:

The DHC-8 Series 400 aircraft is equipped with a Solid-State Flight Data Recording system, 

which is basic to the avionics installation. It comprises of a Solid-State Flight Data Recorder 

(SSFDR), an acceleration (impact) switch, which interfaces with the Flight Data Processing 

System (FDPS) residing within the Input Output Module (IOP), a test circuit, an input power 

interlock arrangement and the flight-deck Caution panel. 

The SSFDR system operates by receiving mandatory and non-mandatory parameters from the 

FDPS and discrete from aircraft systems at a configuration module programmable 512/128 

words per second via an ARINC 717 databus. The unit stores 25 hours of data in a crashsurvivable memory module for subsequent retrieval and analysis. 

The SSFDR design, installation and recording database are compliant with the requirements of 

EUROCAE Documents ED-55 and FAA 14 CFR Part 121.343 and 121.344. 

In order to accommodate additional parameters required by the FAA 14 CFR Part 121 aircraft in 

2010, a Flight Data Recorder Signal Conditioning Unit (FSCU p/n 1152862-X) was designed 

and manufactured by Honeywell to install on the DHC-8 Series 400 aircraft. 

The FSCU was introduced to comply with the FAA 14 CFR Part 121.344 requirement to record 

the following parameters: 

DHC-8 Series 400 Operators 
2 

- Pilot Pitch Control (Column) Position 

- Copilot Pitch Control (Column) Position 

- Pilot Lateral Control (Handwheel) Position 

- Copilot Lateral Control (Handwheel) Position 

- Yaw Control (Rudder Pedal) Position 

The FSCU is included in the basic installation on the DHC-8 Series 400 aircraft and an 

extension of the IOP Flight Data Concentrator (FDC) functionality. The FSCU is a 

microprocessor based box with data control functionality performed by software. It processes 

and formats position data received from sensors installed in the flight controls, and outputs the 

data for recording by the FDR. 

The FSCU provides excitation voltages to the position sensing potentiometers. These sensors 

convert the mechanical movement of the associated control position to a voltage signal, which is 

transmitted back to the FSCU. 

The FSCU processes the voltage signals and converts them into ARINC 429 format for 

transmission to the IOP. The IOP in its function as Flight Data Concentrator (FDC) includes 

these data in the FDR frame format, which are transmitted to the FDR for storage. 

The FSCU system power is supplied through an interlock system. In order to comply with the 

recording requirements of ED-55 and the airworthiness regulations, power to the FSCU is not 

applied automatically when aircraft power is first turned on. The FSCU is enabled only when 

one or more of the following conditions is true: 

- Red anti-collision lights are switched ON, or 

- White anti-collision lights are switched ON, or 

- Both engines are running, or 

- Weight-off-Wheels is sensed, or 

- Cockpit FDR Test switch is set to GND TEST. 

FAA Title 14 CFR Part 121 Amendment:

FAA amended Title 14 Code of Federation Part 121 to revise Section 121.344a (e) and add 

Section 121.346 on April 20, 2010 regarding Flight Data Recorders – Filtered Data. 

Section 121.346 (a) to (c) detailed the affected parameters and criteria regarding filtering. 

Section 121.346 (d) stated the details about compliance time as follow: 

(d) Compliance: 

(1) No later than October 20, 2011, each operator must determine, for each airplane on its 

operations specifications, whether the airplane’s DFDR system is filtering any of the 

parameters listed in paragraph (c) of this section. The operator must create a record of 

this determination for each airplane it operates, and maintain it as part of the correlations 

documentation required by 121.344(j)(3) of this part. 
3 

(2) For airplanes that are not filtering any listed parameter, no further action is required 

unless the airplane’s DFDR system is modified in a manner that would cause it to meet 

the definition of filtering on any list parameter. 

In order to support our DHC-8 Series 400 aircraft operator in the United States to demonstrate 

compliance with Section 121.346(d)(1) by October 20, 2011, BA, together with the FSCU 

supplier, Honeywell, has analyzed the FDR parameters described in Section 121.344(a)(12) 

through (17), (42) or (88), as required by Section 121.346(c) to determine if the recorded signal 

value is filtered and does not meet the requirements of Appendix M of Part 121. 

During BA and Honeywell analysis of the relevant FDR parameters on the DHC-8 Series 400, it 

has been determined that the data collected from the force sensors are fed into the FSCU have 

been filtered by the Flight Data Recorder Signal Conditioning Unit (FSCU) to minimize the 

potential effect of sensor signal noise prior to FSCU ARINC 429 transmission. 

Honeywell responded that the following parameters were filtered by a software based sampling 

averaging algorithm. 

 Parameter (12): Pilot and Copilot Pitch Control Input Position 

 Parameter (13): Pilot and Copilot Lateral Control Input Position 

 Parameter (14): Rudder Pedal Control Input Position 

 Parameter (88): Pilot and Copilot Control Wheel Force 

 Parameter (88): Pilot and Copilot Elevator Control Column Force 

 Parameter (88): Pilot and Copilot Left and Right Rudder Pedal Force 

A Service Letter DH8-400-SL-31-007 dated 9 September 2011 was sent to all our affected 

DHC-8 Series 400 operators in the United States to summarize the results of the FDR 

parameters review and provide all relevant information for recording as per Section 

121.346(d)(1). 

SCHEDULE

Plans for compliance with Section 121.346 (d):

Section 121.346 (d)(3) stated the details about compliance if FDR filtering has been observed:

(3) For airplanes found to be filtering a parameter listed in paragraph (c) of this section, the 

operator must either: 

(i) No later than April 21, 2014, remove the filtering or 

(ii) No later than April 22, 2013, submit the necessary procedure and test results 

required by paragraph (c)(2) of this section. 

It has been determined that the FSCU filters force parameters destined for FDR recording. BA 

has engaged with the FSCU supplier, Honeywell, to examine a solution for removal of the 

filtering from the current design. 
4 

Between January to July 2012, several significant technical impediments and business issues 

were raised. First, it was unclear from the initial analysis if current FSCU software filtering 

functionality could be removed without additional supporting hardware modification. Second, 

previous compliance to FAA 14 CFR Part 121 Appendix M in Year 2010 regulations requiring 

force sensor parameter sampling rate increased from 4 Hz to 8 Hz had consumed the majority 

of spare processing time. Third, removal of the software averaging functionality may allow 

unacceptable sensor noise to be recorded requiring additional logic to remove noise but ensure 

no data lag would add complexity to the design solution. 

Based on the above issues, Honeywell had further reviewed impact to system and commences 

developing three potential solutions: 

 Option 1: Complete Redesign of the FSCU creating a new box 

 Option 2: New design to communicate the yoke position with the Data Recorder 

 

 Option 3: Modification of the existing FSCU 

At the beginning of September 2012, BA and Honeywell were evaluating the option stated 

above. 

Option 1: Complete Redesign of the FSCU creating a new box. New FSCU would have 

simplified signal chain maximizing accuracy and speed. Based on Honeywell relocation during 

development period a sub tier supplier Curtis Wright to design, build and test new FSCU. 

Bombardier considered options as high cost, medium technical but high schedule risk. To be 

considered with option 3. 

 

Option 2: New design to communicate the yoke position with the Data Recorder 

The option of bypassing the FSCU with the yoke force sensor data to increase FSCU spare 

processing capacity. Option proved to be technical complex as data would have to be time 

stamped and merged with remaining FSCU processed data. Bombardier felt very complex 

solution involving multiple suppliers with unknown technical and schedule risk. Option was 

rejected. 

 

Option 3: Modification of the existing FSCU 

FSCU modification if proven viable would have been most cost effective with moderate 

schedule risk however due to technical issues identified was considered too great a risk by BA 

to pursue as a sole means of compliance. Bombardier requested Honeywell to evaluate further 

with option 1. 

November 2012, Honeywell confirmed no hardware modification of the existing FSCU (option 3) 

was required to remove signal averaging function. There was however significant risk of 

unacceptable sensor noise recorded. Bombardier requested Honeywell to develop a modified 

FSCU prototype based on the current box design for aircraft evaluation. 
5 

There is increasing concerned that the FSCU redesign effort by Curtis Wright may not support 

the FAA retrofit campaign by April 21, 2014. 

During first quarter of 2013, a prototype FSCU with modified software is developed by 

Honeywell (option 3). 

Bombardier conducts formal functional and EMI ground based testing of the modified Honeywell 

test unit, April 2013. FDR recorded data is deemed acceptable. Honeywell was informed to 

officially develop, test, certify and proceed with option 3. 

To expedite aircraft fleet implementation Honeywell reviewed the development of an in-field 

FSCU software loading tool. This technique has proven to be difficult to implement by 

Honeywell and is not currently under consideration. This has negatively impacted the aircraft 

implementation schedule as an FSCU rotation pool will be the sole means of aircraft upgrade. 

Since June 2013, an aggressive development schedule supporting certification of new FSCU 

p/n 1152862-5 prior to April 21, 2014 mandate has been generated: 

Honeywell preparation of Drawings, software requirement 

development 

July – August 2013 

Honeywell build production software, prepare test cases, update 

Engineering documents i.e. software accomplishment summary, 

interface control document, version description document etc 

September – October 

2013 

Honeywell building production FSCU October – November 

2013 

Production FSCU and Software verification and validation by 

Honeywell 

October - December 

2013 

Official release of Engineering documents after verification and 

validation, Certificate of Conformity and shipment of production 

unit to BA by Honeywell review software documents, update of 

wiring drawing, preparing modsum package, facilitate customer 

certification testing on aircraft and prepare certification reports. 

December 2013 to 

January 2014 

BA to perform production first article inspection. BA and 

Honeywell to release respective Service Bulletins. 

January– February 2014 

Honeywell update of Component Maintenance Manual. February 2014 

BA update aircraft maintenance manual and parts catalog February 2014 

JUSTIFICATION
6 

Reasons for exemption requested:

Per the schedule above, the new updated FSCU will not be available for aircraft in-service 

retrofit until Feb 17, 2014. 

There are total of 86 aircraft being affected with two major operators in the United States, 

primarily Horizon Air and Republic Airlines. The set up time required removing, replacing, 

closing up and returning to service of the FSCU is approximately 6 hours. The intent is to 

perform 1 upgrade per night at Horizon and Republic’s facilities. There is an additional concern 

that some older in field FSCUs to be upgraded will be required to be brought up to the current 

production standard causing additional rework at Honeywell. This will affect the rotational unit 

pool turnaround times. It is estimated that based on 5 aircraft updates per week 4 months will be 

required to complete the fleet upgrade. 

With the existing schedule it is anticipated that all 86 affected DHC-8 Series 400 will be 

retrofitted to comply with Section 121.346 by July 21st 2014. 

BA’s position:

Therefore, we kindly request FAA for granting our operator with a three month exemption after 

April 21, 2014 on the compliance of Section 121.346. 

If you have any further questions, please contact Christina Yung at 416-375-4038 or 

christina.yung@aero.bombardier.com. 

Regards, 

 

Chris Watkiss 

Manager, Airworthiness – Toronto 
Quality Assurance 

Coordination Memo

Honeywell International Inc. 

This information is considered proprietary and confidential to Honeywell. If you are not the intended recipient, you are hereby notified that any form 

of dissemination of this communication is strictly prohibited. If this information was sent to you in error, please immediately notify us by phone.

1 of 3

Product Name Q400 / Dash 8 FSCU Ref. Number CSB-104980

Customer: Bombardier Initial Issue Date 09/27/2013 

Attention: Issue # / Date Rev 2 11/22/2013

Cord Memo Type Upgrade Campaign Originator: Carlotta Bowman – 

Product Support 

Program Manager, 

Deer Valley 

Status: Open 

Distribution: 

Bombardier: Tudor Madaras Honeywell: Mark Guenther 

Roberto Diaz Atienza 

Marius Dobre 

 

SUBJECT: PM NIC/PROC Module 

Product Information 

Platform/Program Q400 / Dash 8 FSCU Software Change 

Affected Part Name, Part Number FSCU, 1152862-1, -2, -3, and -4 change to 1152862-5 

Affected Sub-detail Name, No. N/A 

Scope of Product Affected

All fielded units (86) in the United States of America are to be upgraded and modification included in all forward-fit 

equipment. 

Description of Non-Conformance

The specification of the existing Honeywell (HW) Flight Data Recorder Signal Conditioning Unit (FSCU) did not include 

requirements which comply with the new FAA regulation (FAR 121.346). With current design, the FSCU is filtering data 

by averaging samples, and this is not acceptable under the new mandate. 

Current software filtering must be removed and the FSCU must continue to meet FAA-mandated acquisition speed 

and accuracy. The FAA Mandate FAR 121.346 amended in 2010 prohibiting filtered flight data being recorded; Affects 

Part 121, 125, 135 operations; compliance required by April 21, 2014. 

System Effects 

FAA considers a signal filtered when an original sensor signal is changed in any way, other than changes 

necessary to: 

• Accomplish analog to digital conversion of the signal 

• Format a digital signal into a DFDR compatible format, or 

• Eliminate a high frequency component of a signal outside the operational bandwidth of the sensor 

The rule requires that, for the flight control related parameters, the DFDR system either: 

• record unfiltered data 

• record filtered data that meet the required accuracy specifications, or 

• record filtered data that can be reconstructed to recover the original unfiltered sensor signal values 

Disposition/Recommendations

This is not a Flight Safety Issue. Honeywell’s plan is to have all (86) units returned to the Toronto Service Center for upgrade. 
Quality Assurance 

Coordination Memo

Honeywell International Inc. 

This information is considered proprietary and confidential to Honeywell. If you are not the intended recipient, you are hereby notified that any form 

of dissemination of this communication is strictly prohibited. If this information was sent to you in error, please immediately notify us by phone.

2 of 3

Root Cause Investigation and Corrective Action(s)

Root cause: 

FAA Mandate: FAR 121.346 amended in 2010 prohibiting filtered flight data being recorded; Affects Part 121, 125, 135 

operations; compliance required by April 21, 2014 

Corrective Action: 

Current software filtering must be removed, but allow the FSCU to continue meeting FAA-mandated acquisition speed 

and accuracy 

Honeywell Actions

Upgrade (86) units, plus include forward-fit equipment. 

Initially it was believed that modification of the current FSCU would be problematic. Therefore Honeywell originally 

investigated a solution which involved the development of a completely new FSCU. However, program leadtime for 

Certification was well in excess of the April 2014 mandated date of compliance. Subsequently, additional review and 

testing of the current FSCU determined that a modification of the Software was possible to meet the new FAA 

requirements. 

 

A FSCU S/W development, certification and field implementation plan has been established and compressed where 

possible without adding un-acceptable risk. However, the overall program leadtime currently exceeds the April 2014 

incorporation date with current program completion by July 2014. 

The following are key program Milestones for reference which commence with the initial testing, which confirmed the 

viability of the change to the current FSCU and concluding in July with the completion of all the aircraft modifications. 

Software Change Implementation: 

Software Solution Accomplished Milestones: 

1) A/C level confidence test was successfully completed – (Apr 2013) 

2) BA sent Memo to HW summarizing test results – (May 2013) 

3) HW submitted ECP – (Jul 2013) 

4) BA approved ECP – (Aug 2013) 

5) HW submitted improved Project Schedule – (Aug 2013)

6) Build & Test Production Verification and Validation – (Oct 2013) 

7) Delivery of HW Black Label – (Nov 2013) 

 

Software Solution Go Forward Milestones/ Next Steps: 

1) BA Aircraft Testing Dec 2013 

2) Rotation Units (10) to Support Rework Campaign Jan 2013 

3) HW Release Service Bulletin Feb 2014 

4) BA Release Aircraft Level SB Feb 2014 

5) Upgrade Campaign Complete Jul 2014 

Upon completion of the engineering development portion of this program, Honeywell will proceed with the following key 

program steps for the implementation of the software change on current in-service units. Once the software change is 

released and approved, it will also be incorporated in production for forward fit equipment. 

1) Release New Software Change 

2) Production and Test Validation and Verification 
Quality Assurance 

Coordination Memo

Honeywell International Inc. 

This information is considered proprietary and confidential to Honeywell. If you are not the intended recipient, you are hereby notified that any form 

of dissemination of this communication is strictly prohibited. If this information was sent to you in error, please immediately notify us by phone.

3 of 3

3) Release Service Bulletin 

1. SB will introduce new FSCU software. Additionally, it will include all concurrent requirements to bring 

units to the currently released -4 configuration. 

4) Upgrade (86) FSCUs 

1. Issue rotable units to operators (6 to Horizon, 4 to Republic) 

2. Removed units will be returned to Toronto SC for software upgrade (covert to -5) 

Upgrade Plan:

References

Preliminary List of Affected Operator Units requiring Upgrade: 

Horizon Total = 51 

-1 units Qty 14 

-2 units Qty 8 

-3 units Qty 16 

-4 units Qty 13 

Republic Total = 34 

-3 units Qty 19 

-4 units Qty 13 

Concurrent Requirements: 

Honeywell SB 1152862-31-01, Modification of P/N 1152862-1 to P/N 1152862-2 

Honeywell SB 1152862-31-02, Modification of P/N 1152862-2 to P/N 1152862-3 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

455555555555555551

4 9 14 19 24 29 34 39 44 49 54 59 64 69 74 79 84 85

33433433433433433433433432

3 6 10 13 16 20 23 26 30 33 36 40 43 46 50 53 56 60 63 66 70 73 76 80 83 85

Tuesday Operator ships exchanged parts to Honeywell for upgrade Operator Action

Wednesday Honeywell ships upgraded parts to Operator Honeywell Action

QTY: 10 Rotable Units (6 Horizon & 4 Republic)

Week Day Description

Monday & Tuesday Toronto R&O upgrades units received earlier week Honeywell Action

Nominal Case

Thursday & Friday Toronto R&O upgrades units received on Tuesday Honeywell Action

Friday Operator ships exchanged parts to Honeywell for upgrade Operator Action

Total Units

Worst Case

Total Units

WEEK

Saturday Honeywell ships upgraded parts to Operator Honeywell Action

1 Cycle = 6 Days