B727 & B737 Window Heat Controller

 

A) Reason for the study

 

1)       B727/737 Window heat controller unit (WHCU) p/n 231-2 (alt p/n 65-52803-8, 83000-05602, 10-61833-2) was among the top 5 unscheduled component removal for 4 consequences months from Nov 2007 till Feb 2008.  These studies analyze common reasons of such failures from year 2005 till Nov 2007 to enhance its reliability.

2)       There are various P/Ns (various OEMs) of Window Heat Controller Unit (WHCU) installed on B727/B737. All P/Ns are fully interchangeable.

P/N

Boeing P/N

OEM

231-2

Nil

Astronic

83000-05601 / -05602

10-61833-2

Koito

65-52803-8

Nil

BAE Systems

3)       There were 4 units of WHCU installed on 727 (and B737) airplane which for pilot and co-pilot No. 1 and No. 2 windows. The WHCU located at E5-1 electrical rack. It consists of temp controller which a solid state device that performs overheat control and temp control, overheat relay which direct 115V AC power from window heat CB to temp controller when energized and transformer which provide high voltage for heating window.

 

 

B) Data

1)       Repair and Findings Data from 1st Jan 2005 till 31st May 2008 were reviewed and has been classified into various types of common defect and shop findings; s/n and aircraft with repeated removals. Total 43 units were removed unscheduled with 30 DC and 13 DNC. None were scrapped or overhauled.

Defect Confirm

Defect Not Confirm

30 (69.8 %)

13 (30.2%)

2)       Unscheduled removals unit in 2005, 2006, 2007 and 2008 (till May) are shown below:

Year

2005

2006

2007

2008 (till May)

Removal Units

12

10

19

2

3)       MTBUR for B737 is 2935 hrs and for B727 is 4112 hrs. The average MTBUR is 3838 hrs. Design MTBUR for WHCU p/n 83000-05602 is 14046 hrs while MTBF is 29200 hrs.

C) Current Maintenance Program

B737

B727

Description

Every ‘C’ chk (Card: C-102A-2)

Every ‘C’ chk            (Card: 72C1-E-2-4-016)

Inspect cabin control window anti-icing syst for components such as window heat power relay, anti-ice control panel, heat conductive coating, thermal switches and heat sensors for security, wiring condition and evidence of overheat.

Every ‘C’ chk (Card: C-103A-1)

 

Inspect window heat control unit (4 places) installed on the E3 rack for security of installation, condition of wiring, cleanliness and evidence of overheat / moisture.

 

D) OEM comments.

Email from Boeing and OEM are described below:

1)        Boeing via email dated April, 30 and May, 9 provided comments per below:

  1. a) Boeing has not received any reports from other operator similar to WHCU defects.
  2. b) Boeing provides current MTBF reported from Koito is 29200 hrs.
  3. c) Boeing has limited knowledge of the Astronics p/n 231-5 due to this is an STC mod and therefore the p/n is not reflected in Boeing IPC.
  4. d) There is no BAE p/n that equivalent to Boeing improved p/n 10-61833-6.
  5. e) Boeing recommends to upgrades WHCU to the newest Koito p/n 83000-05604 (Boeing p/n 10-61833-6). Boeing drawing provides data allowing this p/n to be installed to B727 airplane. The IPC rev July 2008 will reflect the p/n 83000-05604 usage on 727-200 airplanes.
  6. a) OEM does not track the MTBUR and MTBF of Window heat controller unit p/n 231-2.
  7. b) 231-5 is the latest WHCU p/n produce by Astronics. It is fully interchangeable with p/n 231-2 with an addition of BITE circuits.
  8. c) Suspect the latest mods (Mod L or M depending on the age of the unit) have not been  incorporated for serial number that have failures of parts in the output transistor section (Q26, 27, 28)
  9. d) Astronics has produced 16 SBs related to WHCU p/n 231-2. Refer attachment 7B for modification history from p/n 231-1 to 231-2 mod ‘M’.
  10. e) Astronics only do repair and re-certify origin WHCU p/n 231-x from astronics or all predecessor company names for astronics. 
  11. f) Astronics provide recommendation per below:
  12. Due to cooling air system accumulates dust and dirt which creates thermal stress on the unit, operator is recommended to review the maintenance chk task to inspect the cooling system and cleaning the dust and dirt in E&E compartment to keep dust from clogging the units.
  13. Operator to record mod level for each WHCU installed on the fleet.
  14. Any units had RV1-4 or F1 changed should have Q1 changed as well or there is risk of recurrence of the problem.
  15. b) Since most of the operators has already incorporated new WHCU p/n: 83000-05604, there is no news about p/n 83000-05602 recent few years.
  16. c) P/n 83000-05604 has a BITE function while -05602 has not. The other differences are the weight of unit. -05604 weight is 4.3 kg while -05602 is 3.7 kg.
  17. d) Koito recommended below:
  18. To upgrade p/n 83000-05602 to -05604 (Boeing pn 10-61833-6). Info: p/n 83000-05604 is interchangeable with -05602. P/n 83000-05602 is no longer produce by Koito.
  19. Send repair or overhaul’s WHCU to Koito repair station or Aviation Technical Services, Inc (formerly Goodrich ATS).
  20. All new purchase of Koito WHCU must be obtained from AAxico.
  21. a) No expected MTBUR and no recommended improved part number from repairer.
  22. b) Other operators do not have low time failure (LTF) for window heat controller p/n 231-2.
  23. c) Two units (S/n 478 & 6205) LTF was warranty denied.

2)    Comments from Astronics Advanced Eletronics Systems (previously known as General Dynamics or Olin or Pacific Electro Dynamics)

3)    Comments from Koito Mfg

a)        Design MTBUR for WHCU p/n 83000-05602 is 14046 hrs.

4)    Comments from Aero Technology

1)        S/n 478 was repaired at 1st time visit.  Nil faults found for 2nd shop visit.  

2)        S/n 6205 found internal damaged due to excessive heat on Oct 07. 1 month later the unit was sent for repair and found different circuit board had failed. It appears that the second failure was also caused by an excessive heat.

  1. d) Unable to offer exchange with upgraded p/n due to no stock available.

E) Findings and Discussion.

1)       16 out of 43 WHCU unscheduled removals were caused by overheat. From the study, overheat will affect the window and cause the window to crack. 5 unscheduled removals due to CBs tripped can also cause window to overheat.  

Reason for Removals

%age

Total

DC

DNC

Overheat / fail overheat test

37.2

16

13

3

Nil heating / inop

25.6

11

9

2

Nil control / nil indication / not regulating / not function

11.6

5

3

2

CB tripped

11.6

5

2

3

Window cracked / arching

9.3

4

1

3

Green light intermittent

4.7

2

2

0

 

Total

43

30

13

 

2)       29 (67.4%) units were sent to Aero Tech for repair, 6 (14%) to Aero Instrument, Avborne 3, High Tech Avionic 1 and Aero Control Avionic and ST Aero 2 each. Most of units were sent to Aero Tech due to low flat rate compare to other vendor.

 

Aero Tech

Aero Instrument

Aero Control Avionic

Avborne

ST Aero

High Tech Avionics

Total

2005

4

1

2

3

2

12

2006

9

1

10

2007

15

4

19

2008

1

1

2

Total

29 (67.4%)

6 (14%)

2 (4.65%)

3 (7%)

2 (4.65%)

1 (2.3%)

43

3)       Currently there were 57 units installed on the fleet which 47.4% was manufactured by Astronics, 33.3% by BAE system and 17.5% by Koito.

4)       The S/Ns show the repeated removals are 478 (4 removals), M01372 (3 removals), 662, 1607, 1634, 3718, 5126, 6205, 7142 and 7145 (2 removals each s/n).

5)       Most of the WHCU (47.4%) belongs to Astronic. However, Boeing has limited knowledge of the unit and cannot determine the replacement of part 231-2 with 231-5.

6)       All of Astronics p/n 231-2 was not upgraded to latest mod L or M.

7)       Koito unit is the most recommended p/n due to 50% of the unit is in good condition and only one was removed unscheduled for the last 1 year. Furthermore, Boeing only recognizes Koito p/n compared to Astronics or BAEp/n.

8)       BAE p/n 65-52803-8 had shown only 5 units removed unscheduled last year (2007). Compared to Astronics and Koito, BAE WHCU reported fewer problems. However, the OEM (BAE) is NOT contactable. 

F) Recommendations

1)       To advice repairer;  unit found with varistor (RV1-4) or fuse (F1) defect/damaged, transistor Q1 must be replaced to prevent from tripping and overheat shutdown

2)       To evaluate upgrading Koito WHCU p/n 83000-05602 to the newest p/n 83000-05604 (Boeing p/n 10-61833-6)

3)       S/n 7097, to incorporate mod K at next shop visit

4)       S/ns 662, 2749, 3486 and 7142 to incorporate latest mod L or M at next shop visit.

5)       Close monitoring for WHCU s/n: 478, 6205 and M01372 that have low time failure (LTF) last year. Scrapped or exchange for those s/n that have more than 3 LTF within 2 years (until June 2009).

6)       Should spares need to order extra WHCU, always purchase latest version pn: 83000-05604 alt pn: 10-61833-6 from Aaxico (Koito prefer seller).

7)       Inform the maint crew to follow the troubleshooting chart in AMM 30-41-02 figure 101 before make a decision to replace the WHCU due to failure. Ensure that the window temperatures are below 75°F and No. 2 windows are closed and latched before using the troubleshooting chart.

8)       Review maintenance task (‘C’ chk and ‘B’ chk) to inspect the cooling system and cleaning the E&E from dust and dirt.  

G) Conclusions

1)     Operator Window Heat Controller Unit (WHCU) MTBUR is 3838 hrs which is lower than the design MTBUR (14046 hrs).

2)     Aging is the main reason of the unscheduled removals.

References:

Email from Aero Tech dated 22, 29 April, 28 May 2008

Email from Astronics dated 6 and 7 May 2008

Email from Boeing dated 30 April, 9 and 31 May 2008

Email from Koito dated 23 May 2008, 14 and 17 June 2008

Email from Aviation Technical Services dated 24 May 2008

Email from Aaxico dated 26 May 2008

ISAR No. 93-10 dated 30 September 1993

ISAR No. 90-11 dated 12 December 1990

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