Discussion
What events have caused this AD? Reports of the vent blower assembly bearings seizing and locking the blower motor on several Raytheon Model 1900, 1900C, and 1900D airplanes caused us to issue AD 97-22-16, Amendment 39-10187 (62 FR 58894, October 31, 1997. AD 97-22- 16 currently requires the following on certain Raytheon Model 1900, 1900C, and 1900D airplanes:
--Incorporating a modification to replace the bearings in the vent blower assemblies with improved design bearings (Electromech Technologies Kit No. EM630-201-1 or EM630-201-2 (as appropriate for the blower serial number)); and
--Installing a thermal protection for the vent blowers (Electromech Technologies Kit No. EM630-201-1 or EM630-201-2 or Advanced Industries Kit No. BC80A905 (as appropriate for the blower serial number)).
What has happened since AD 97-22-16 to initiate this action? The FAA has received reports that vent blower assemblies modified in accordance withAD 97-22-16 are still malfunctioning.
What is the potential impact if FAA took no action? If not corrected, smoke could enter the cockpit and cabin, which could result in the pilot becoming incapacitated or impairing her/his judgment. This condition could lead to the pilot not being able to make critical flight safety decisions and result in loss of control of the airplane.
Has FAA taken any action to this point? We issued a proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an AD that would apply to certain Raytheon Model 1900, 1900C, and 1900D airplanes. This proposal was published in the Federal Register as a notice of proposed rulemaking (NPRM) on June 4, 2003 (68 FR 33420). The NPRM proposed to supersede AD 97-22-16 with a new AD that would retain the actions required in AD 97-22-16 for certain vent blower assemblies and require you to incorporate further product improvement modifications for all affected vent blower assemblies.Comments
Was the public invited to comment? We provided the public the opportunity to participate in the development of this AD. The following presents the comments received on the proposal and FAA's response to each comment:
Comment Issue: The Proposed AD Does Not Solve the Problem of Smoke in the Cockpit/Cabin
What is the commenter's concern? The commenter states that the proposed AD focuses on the vent blower low speed resistors and does not consider the motor as a source of smoke. Since the cooling air for the motor is drawn through the motor by the blower intake and then is exhausted into the cabin, any motor failure that generates smoke and fumes is blown directly into the cabin.
The following summarizes the commenter's concerns:
--The insulation on the lead-wire can hang up on the brush holder if the brush lead-wire is pushed down to clear the metal brush inspection cover. The installation instructions provided with Modification Kit BC80A-901-3, which incorporates the replacement brushes with the lead- wire insulation, do not clarify where the lead-wires should be formed;
--When a brush lead-wire gets hung up on the brush holder (caused by improper lead-wire forming), there is little to no force from the spring to hold the brush against the commutator. Lack of force to hold the brush against the commutator can cause arcing between the brush and commutator surface, which produces tremendous heat and accelerated brush wear. This will ultimately lead to excess heat that will cause the grease to boil out of the bearings and result in bearing failure;
--The hard anodized coating on the metal brush inspection cover provided with Modification Kit No. 630-203-1 is a poor insulator and is easily scratched during assembly and disassembly. Once scratched, the coating offers no insulation, which creates the possibility for a short circuit to the brush shunt; and
--The low speed power resistors are considered undersized for the application.
The commenter recommends the following:
--Installing thermal fuse(s) on the motor to interrupt the current in the event of the motor overheating;
--Controlling how the brush lead-wires are formed to prevent shorting to metal inspection screen or any other adjacent conductor; and
--Increasing the power rating of the low speed resistors to improve the safety margin.
The commenter requests these changes based on personal repair history as well as analysis of the design.
What is FAA's response to the concern? We do not agree. Although the commenter raises many pertinent concerns, we consider the requested changes a product improvement or a way to increase the reliability of the motor.
The proposed AD is intended to address smoke in the cabin/cockpit that is specifically caused by the vent blower assembly. Investigation by Raytheon engineering concluded that all incidents involving smoke in the cockpit/cabin were caused by overheating of the vent blowerlow speed resistors while operating the blower on low speed.
AD 97-22-16 required incorporating the applicable modification kit as specified in Raytheon Service Bulletin No. 2721, Issued: January, 1997. Raytheon Service Bulletin No. 2721 added a 216oC thermal cutout to the resistor assembly to interrupt power to the resistors and prevent overheating.
Further field experience revealed that the 216oC cutout may not open soon enough to prevent overheating in all instances. As a result, Raytheon issued Mandatory Service Bulletin SB 21-3448, Issued: October, 2002, to decrease the thermal cutout set point to 152[deg]C. New tests verified that this value cutout to the resistor assembly provides adequate protection against resistor overheating while avoiding nuisance trips during normal operation.
The addition of an insulating sleeve over the brush lead-wires and hard anodizing of the brush inspection cover required by Raytheon Mandatory Service Bulletin SB 21-3448, Issued: October, 2002, was done to offer an additional measure of protection. The additions are not meant to substitute proper brush lead-wire routing.
The thermal cutout on the resistors and the aircraft's blower circuit current limiter are the primary methods of protection. If a brush lead-wire was shorted to the housing and the vent blower was operated in the low speed mode, the increased current flow would cause the resistor temperature to increase until the thermal cutout opens and interrupts power to the resistors. If a brush lead-wire was shorted to the housing and the vent blower was operated in the high speed mode, the increased current flow would cause the aircraft's current limiter to open and interrupt power.
A brush lead-wire could possibly get hung up with or without the sleeving. If the brush lead-wire gets hung up, this would result in arcing between the brush and commutator resulting in increased heat and accelerated brush wear until the blower no longer continues to operate. At this time, the manufacturer has not received any field reports of smoke related to "hung brushes."
The power dissipation in the low speed circuit does appear to be above the rated value for the resistors. However, this does not account for the large amount of cooling airflow that passes over the area to which the resistors are mounted. Service history shows that the resistor rating is adequate under normal operations. If the resistors start to overheat because of vent blower failure, then the thermal cutout will open and interrupt power to the resistor assembly.
Since none of the recommendations specifically address an unsafe condition, we have determined that we are not changing the final rule AD based on these comments.
Conclusion
What is FAA's final determination on this issue? We have carefully reviewed the available data and determined that air safety and the public interest require adopting the AD as proposed except for minor editorial corrections. We have determined that these minor corrections:
--Are consistent with the intent that was proposed in the NPRM for correcting the unsafe condition; and
--Do not add any additional burden upon the public than was already proposed in the NPRM.
Changes to 14 CFR Part 39--Effect on the AD
How does the revision to 14 CFR part 39 affect this AD? On July 10, 2002, the FAA published a new version of 14 CFR part 39 (67 FR 47997, July 22, 2002), which governs the FAA's AD system. This regulation now includes material that relates to altered products, special flight permits, and alternative methods of compliance. This material previously was included in each individual AD. Since this material is included in 14 CFR part 39, we will not include it in future AD actions.
Costs of Compliance
How many airplanes does this AD impact? We estimate that this AD affects 300 airplanes in the U.S. registry.
What is the cost impact of this AD on owners/operators of the affected airplanes? We estimate the following costs to accomplish the modification:
Labor cost
Parts cost
Total cost per airplane
Total cost on U.S. operators
3 workhours x $60 per hour = $180
$415 (for both the forward and aft ventilation blower assemblies).
$595
$595 x 300 = $178,500.
Regulatory Findings
Will this AD impact various entities? We have determined that this AD will not have federalism implications under Executive Order 13132. This AD will not have a substantial direct effect on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government.
Will this AD involve a significant rule or regulatory action? For the reasons discussed above, I certify that this AD:
1. Is not a "significant regulatory action" under Executive Order 12866;
2. Is not a "significant rule" under the DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979);and
3. Will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act.
We prepared a summary of the costs to comply with this AD and placed it in the AD Docket. You may get a copy of this summary by sending a request to us at the address listed under ADDRESSES. Include "AD Docket No. 2003-CE-16-AD" in your request.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.
Adoption of the Amendment
Accordingly, under the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:
PART 39--AIRWORTHINESS DIRECTIVES
1. The authority citation for part 39 continues to read as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701.
39.13 [Amended]
2. FAA amends 39.13 by removing Airworthiness Directive (AD) 97- 22-16, Amendment 39-10187 (62 FR 58894, October 31, 1997), and by adding a new AD to read as follows: