World Meteorological Organization

Supplemental Type Certificate (STC) 

The information contained in this document is provided for explanatory purposes only, and does not represent official policy for any aviation regulatory agency.  The discussion of the Water Vapor Sensing System (WVSS-II) is used throughout to answer specific questions and provide a better understanding of the processes and status of STCs related to WVSS-II for use in global AMDAR operations.  A similar process can be expected for any aircraft type modification associated with other sensors or new technologies.

What is an STC?

A Supplemental Type Certification, STC, is issued by an appropriate aviation regulatory agency to grant approval for an alteration of an aircraft by a major change in the aircraft type design, where such a change is not large enough to require a completely new application for a Type Certificate.  The STC is held by the applicant and is then the basis for issuing or retaining airworthiness certificates for all aircraft subsequently modified in the same way.  Product Certificates of Conformance are delivered with the modification kits associated with the STC.  In the case of alterations by the original aircraft manufacturer, approval is normally in the form of an amendment to the original Type Certificate, rather than an STC.

Why is an STC Necessary?

An STC is required by all aviation regulatory agencies to ensure that any modification to an aircraft type design is done with approved components and conducted in accordance with the required procedures to maintain the safety and integrity of the aircraft design.  The process is designed to ensure that any modifications to the aircraft do not negatively impact aircraft structural systems, electrical systems, EMI, vibration, icing, or any other aspect that could compromise aircraft airworthiness.

What is the Difference between an STC and a Service Bulletin?

A Service Bulletin is a document issued by the aircraft (or engine or other device) manufacturer to notify the owner or operator of an aircraft about recommended modifications (or required by Airworthiness Directives), substitute parts, special inspections/checks, reduction of existing life limits, or establishment of first-time life limits.  Service Bulletins may or may not be approved by the appropriate aviation regulatory agencies.

What is a PMA and Why is it Necessary?

A Parts Manufacturer Approval, PMA, is required for any supplier to manufacture and deliver a component for an existing STC.  The appropriate aviation regulatory agency will regularly inspect and approve a manufactures facility, Quality Management System (QMS), and other aspects with respect to their ability to manufacture a specific part covered by an STC.  Until a PMA is issued, the part may not be shipped from the manufactures facility or received by the target air carriers.

One exception to this rule is the initial delivery of component subassemblies for use in the first Installation for Conformity, as part of the STC process as outlined in the section "Process for Obtaining an STC".  With the appropriate Aviation 8130-3 forms from the DER, these subassemblies may be delivered to support the first Installation for Conformity of the top level kit assembly for the STC.

Process for Obtaining an STC

The STC process is designed to develop and rigorously test all components of a product design to ensure the continued safety of the aircraft type upon which it will be installed.  For purposes of clarity, we will use the U.S. FAA STC process for the WVSS-II as an example in the following explanations. The STC process for other National/Regional aviation regulatory bodies is very similar, but may deviate in some details.  In the development of an STC, independent technical representatives authorized by the FAA to approve technical designs and installations are used.  These include Designated Engineering Representatives (DER) and Designated Airworthiness Representatives (DAR).

The basic steps to achieve an STC are: Product Technical Design, Product Design Technical Review and Approval, Compliance Documentation Development, Compliance Documentation Review and Approval, Integration of Installation Procedures into Air Carrier Maintenance Procedures, First Installation for Conformity, Final Submission of Data Package to FAA, FAA Approval and Issuance of the STC, and FAA Approval of a PMA.

It should be noted that documentation submitted to aviation regulatory agencies typically has a format and content required by that agency.  It is important to ensure that submitted documents comply with these requirements in order to avoid continuous rework.  This will cause extensive delays, extra engineering work, and frustration by the regulating agency team.  It should also be noted that requirements can change over time, and each individual regulating agency reviewer may have slightly different interpretations on certain aspects of what is required to be included in various documents.

Original STCs

When an original STC is conducted for a new system, all components of the system must be designed, thoroughly tested, reviewed, and approved.  The original STC for WVSS-II was included extensive testing of all subassemblies of the system; the System Electronics Box (SEB), the Air Sampler, the Inlet and Outlet Hoses, as well as Structural Brackets and Electrical Harnesses for the initial aircraft type.  This process includes the following steps.

1.      Project Start Up

The first step is to develop a Project Specific Certification Plan (PSCP).  The PSCP is then submitted to the FAA for review and approval.  Upon approval of the PSCP, a Project Number is assigned for the STC development.  All subsequent interaction with the FAA references this Project Number.

2.      Product Technical Design

This step is the technical development of the product to be installed.  This is similar to the technical development of any other product, but must be done with formal processes including documentation release and control per an FAA approved Quality Management System (QMS).

For WVSS-II, the technical design includes the subassemblies:

• SEB (common to all aircraft types)
• Air Sampler   (common to all aircraft types)
• Heated Inlet Hose   (common to all aircraft types)
• Unheated Outlet Hose   (common to all aircraft types)
• Structural Mounting Brackets (unique to each aircraft type)
• Electrical Harness (unique to each aircraft type)

For purposes of the certification process, these are generally grouped into a second level “Structural Assembly” and “Electrical Assembly” each of which are aircraft type specific.  The final STC approval will be for a single Top Level Assembly, or Installation Kit, that is aircraft type specific, and includes all components.

3.      Product Design Technical Review and DER Approval

Upon completion of the technical design, all drawings must be reviewed and approved by an FAA authorized DER.  Any subsequent changes to this approval require a formal documentation audit trail and documentation release and control per an FAA approved QMS.

4.      Compliance Documentation Development

In addition to the product technical design drawings, all compliance testing must be completed, documented, and submitted for FAA approval.  It should be noted that these documents typically require multiple internal reviews and iterations to be completed.  Some documents, such as installation procedures, maintenance procedures, etc., must also receive one or more reviews by the partner airlines staff to ensure they can meet the needs of the airline.  This includes documents such as:

• Project Specific Certification Plan (PSCP)
• Master Drawing List (MDL)
• Structural Installation Procedures
• Electrical Installation Procedures
• Ground Functional Test Plan
• Ground Functional Test Report
• Electrical Load Analysis
• No Ignition Analysis
• EMI Test Plan
• EMI Ground Report
• Hazard/Safety Report
• Icing Analysis Report
• Flammability Report
• Structural Substantiation Report
• Vibration and Buffett Analysis Report
• Damage Tolerance Report
• Aircraft Flight Manual (AFM) Supplement Report
• Weight and Balance Report
• Installation Functional Test Procedure – Certification Install
• Installation Functional Test Procedure – Subsequent Installs
• Instructions for Continued Airworthiness (ICA)
• ICA Electrical Wiring Interconnection Systems (EWIS)
• Structural Repair Manual (SRM) Supplement
• SEB Technical Report
• Air Sampler Technical Report

5.      Compliance Documentation Review and DER Approval

All the Compliance Documents must be reviewed and approved by an FAA authorized DER.  Any subsequent changes to this approval require a formal documentation audit trail and documentation release and control per an FAA approved QMS, and subsequent approval.  This is an ongoing engineering maintenance requirement that must be carried out for the life of the STC.

6.      Integration of Installation Procedures into Air Carrier Maintenance Procedures

This step involves the collaboration between the STC applicant and the initial air carrier participating in the STC process.  The final installation documentation and drawings are provided to the partner air carrier by the applicant.  The air carrier then fully integrates these into their regular maintenance procedures for that aircraft type, with any internal review and approval necessary.

7.      First Installation for Conformity

The STC process requires that the new system or part be fully installed on an aircraft and subsequently inspected by an FAA approved DAR for “conformity” to the drawing set.  The installation is done by the air carrier maintenance staff according to the air carrier internal procedures developed using the applicant’s technical data.  Engineering staff of the applicant are typically on hand to answer any questions by the air carrier installation team, and make final clarifying modifications of the drawings as necessary to complete the installation.  This is typically done when the aircraft is out of service for a routine “heavy maintenance.”

Upon completion of the installation, the aircraft is inspected by an authorized DAR to ensure the installation conforms exactly to the engineering technical documentation.  It is critical that the installation conforms 100% to the technical documentation.  This is where the term “Installation for Conformity” comes from.

After the physical installation is complete, the system EMI testing must be done to ensure no interference with aircraft avionics.  This test can only be done once the aircraft is ready for a full engine test.

Once the installation is complete, inspected and approved by the DAR, and EMI testing is complete, the WVSS-II system is deactivated at the main circuit breaker until final approval is granted by the FAA.  Appropriate aviation approval forms are provided to the air carrier to allow the aircraft to return to service without the system in operation, before the STC is issued.

8.      Final Submission of Data Package to FAA

After the Installation for Conformity is complete, the full data package is prepared by the applicant for submission to the FAA.  This includes all drawings from the technical design, compliance reports from step 3 and 4, and associated approval forms from the DAR.  This data package is generally several hundreds if not thousands of pages in length.

9.      FAA Approval and Issuance of the STC

Upon submission of the data package, the FAA will review all documentation in the package.  This includes multiple organizations within the FAA structure, responsible for various elements of the aircraft airworthiness.  Questions and clarifications are sometimes necessary.  Upon resolution of all outstanding issues, the FAA will issue the STC, or an amendment to an existing STC, to cover a new aircraft type for the part.  Occasionally this may include restrictions or limitations required by the FAA. 

The time from submission of the STC data package to issuance of the STC is typically 8 to 12 weeks, but could be less or more depending on FAA workload, similarity to prior STCs and other factors out of the control of the applicant.

Upon issuance of the STC, the applicant will provide the air carrier copies of the STC documentation.  The air carrier that supported the initial installation for conformity may then reactivate the WVSS-II installed on the first aircraft.  The STC allows the air carrier to install the system on additional aircraft of the approved type.

10. FAA Approval of a PMA

After the STC is issued from the FAA, the applicant submits the appropriate documentation to the FAA to apply for approval to manufacture the top level kit approved by the STC.  The FAA reviews this application and issues a PMA, or a supplement to an existing PMA.

The time from submission of the PMA data package to issuance of the PMA is typically about 2 weeks, but could be less or more depending on FAA workload, similarity to prior PMAs and other factors out of the control of the applicant.  While the STC granted in step 9 allows an airline to install the system, the manufacturer cannot ship the system until the PMA is granted.  Therefore, in reality it is the PMA that is the final step in the process.

Subsequent STCs for the Product on other Aircraft Types

For STCs that are subsequent to the original STC, all steps of the STC/PMA process are identical to that defined in the section "Original STCs".  However, some technical design, analysis and testing may be used from the previous STC in whole or with minor modifications.  For example with WVSS-II, the SEB, Air Sampler, and hoses are common among all implementations, and therefore much of the analysis from the original STC may be used for subsequent applications with little or no changes.  Only unique new components must be evaluated in extreme detail.  For WVSS-II that includes the Structural Mounting components, the Electrical Harness components, and the Compliance documentation, which are unique to each aircraft type.  This reuse of original documentation and compliance reports greatly reduces the time and cost of subsequent STC applications.

For WVSS-II the majority of technical design work of new STCs will be in developing the necessary technical design for the Structural Mounting Bracket assembly for the SEB and the Electrical Harness assembly for the aircraft type.  These items are unique to each aircraft type configuration.

What does "Owning" an STC mean?

Anyone can apply for an STC, and be granted ownership of the STC.  However, an equipment supplier wishing to produce equipment to be installed on an aircraft type will typically be the applicant for an STC.  They are usually the only ones with sufficient engineering expertise and technical documentation on the product, and are able to maintain the associated records for continued compliance.

Owning the STC means that the applicant has been granted the FAA approval to have their specific equipment design installed on aircraft of the approved type.  Owning an STC also requires the ongoing maintenance of engineering records and various other functions.

Owning an STC has nothing to do with sources of funding used for the STC development.  Various contractual mechanisms may be used to support the technical design development, associated compliance documentation development, submission for STC approval, and continued maintenance of the STC documentation.

What is Required by the STC Owner?

Once and STC is granted, the owner of the STC is responsible to maintain all technical design records related to the STC, maintain records of aircraft owners/operators who are using that STC and keep them informed of any changes in installation or compliance documentation, and regularly notify the FAA of any minor changes to the STC data package.  Major changes to the STC design or any changes to the Instructions for Continued Airworthiness require submission to the FAA for additional review and formal approval by the FAA before they can be implemented.

What does "Holding" an STC mean?

Holding an STC is the same as Owning an STC and the requirements are the same.

Who is the User of an STC and What is required by the STC User?

The STC Users are the air carriers that have installed the approved system or part on their aircraft.  They collaborate with the STC owner, to get all the appropriate documentation regarding the STC, including installation and maintenance procedures.  The air carrier must also integrate the installation and maintenance procedures into their internal operational aircraft maintenance procedures.  The air carrier, or their designated subcontractor, generally performs all installation and maintenance of the approved system.

In the case of WVSS-II, air carriers perform all direct aircraft installation or aircraft maintenance.

Transferring an Existing STC

Transferring an STC between Air Carriers

An STC is not transferred between air carriers.  Once an STC is granted, any air carrier may install the approved part on that aircraft type, provided they follow all requirements and restrictions of the STC and maintain all appropriate records of the aircraft modification.  Typically, this is done by receiving a data package from the Owner of the STC, and integrating the product installation and maintenance procedures into the internal aircraft maintenance procedures of the air carrier.

If there is air carrier specific technical documentation from the aircraft manufacturer that must be evaluated for potential conflict to the design, the air carrier must approve the release and use of that documentation to the STC holder through a standard process.  This would include any specific options or configurations that may be included on the aircraft type by that air carrier.  For example such air carrier specific options may include various passenger entertainment options or galley configuration options that may impact the approved WVSS-II wiring routing.  There may be associated cost to the STC owner for acquiring this carrier specific documentation from the aircraft manufacturer, and for modifications of the design package to accommodate the air carrier specific items on that aircraft type.

Transferring an STC between Countries

The aviation regulatory agencies of the world collaborate closely on STCs, in accordance with the ICAO standards and regulations.  In general terms, if an STC is approved in one country/region, subsequent regulatory agencies may accept all or portions of the technical documentation, require some level of modifications, and approve ( or "validate") the STC for their area of authority.  For example the Owner of an STC may request that the FAA share the technical data package with EASA, CAAC, or CASA, etc.  The receiving agency will typically review the documentation package and accept it as is, or require one or more changes to the design and data package.  Once the changes are completed, they are resubmitted for Review.  Upon approval, they may grant the STC or validate the existing STC for their area.  Any changes to the STC data package must be reviewed and approved by the original agency granting the STC (e.g. the FAA) through the standard process.  This process will work in most directions, FAA to/from EASA, CASA to/from FAA, etc.  But this is entirely dependent on the regulations and processes of the various aviation regulatory agencies involved.  This process will still need interaction by approved DER/DAR staff to facilitate the interaction with the appropriate regulatory agencies.  Generally speaking, the first STC for a specific aircraft type will be the most difficult, and subsequent international STCs for that type will be able to reuse much of the documentation set.  Some aviation regulatory agencies require a fee for the application to approve/validate an existing STC.

Costs Associated with Obtaining an STC

The costs associated with obtaining and maintaining an STC are divided between the Product Supplier and the Air Carrier.  Other costs may be associated with the management and coordination of all parties involved with the STC process.  Such management activities may be addressed by the appropriate NMHS or a designated Program prime contractor.  The STC ownership is not connected with the source of funding used for the STC development.  Various contractual mechanisms may be used to support the Product Supplier, Air Carrier, and other costs associated with obtaining and maintaining an STC.

Product Supplier Costs

For an STC on a new aircraft type the product supplier will be the largest overall cost.  They will have engineering costs associated with: the design and development of aircraft type specific elements of the product, development of all aviation compliance documentation described in the section “Process for Obtaining an STC”, DAR/DER approval of documents and components, manufacturing of the product itself, internal engineering project management, and long term control and maintenance of all design documentation under an approved Quality Management System.

For WVSS-II, the only remaining product development engineering costs are those associated with the design of the aircraft type specific components, which are the Structural Mounting Bracket, Electrical Harness, type specific installation procedures, and associated compliance documentation.  These are unique to each aircraft type and must be designed and developed for each STC.  The aviation compliance documentation, approvals, and management must be done for each STC.

There is long term recurring costs for the ongoing maintenance of engineering drawings and documentation, and required regular reporting to the aviation regulatory agencies.  This includes any changes to specific drawings, and upkeep of the Master Drawing List(s) to reflect the current revision numbers for all drawings in the product.  Typically any change of a drawing requires the additional reporting to all aviation regulatory agency approving or validating the STC in accordance with the STC Management Plan submitted with the PSCP.  Minor changes are generally reported to the regulatory agency on a regular interval, every 6 months.  Major changes must be reviewed and approved by the regulatory agency prior to implementation.

Air Carrier Costs

Air Carrier costs are primarily for the review of engineering drawings and documents from the Product Supplier, integration of those documents into internal airline procedures, installations, limited maintenance, and internal project management.

Development of Internal Procedure

Air Carriers will have costs associated with review and input to installation and maintenance documentation developed for the STC.  This data must also be integrated into the air carrier standard maintenance procedures, which requires non-recurring engineering effort.

Installations

Once all documentation is reviewed and integrated into internal procedures, there is air carrier costs associated with each installation of the product.

For WVSS-II, the average labor for each installation is estimated to be between 60 and 80 man-hours.  The costs will depend on the air carrier maintenance staff expertise and training, as well as hourly rates for maintenance and supervisory staff.

Maintenance

Any regular maintenance of the installed product, as defined in the aircraft type specific Instructions for Continued Airworthiness developed as part of the STC process, must be performed by the air carrier.

The WVSS-II is designed for long term use without calibration, for periods of as much as 6 years or longer.  Some units have been in operation for as much as 10+ years without requiring service of any kind.  Therefore there are minimal costs associated with routine maintenance of the WVSS-II.  Only routine General Visual Inspections are required at intervals required by the aviation authority responsible for that STC.  This is typically intervals such as every 36 months or 9,000 flight cycles.  The Air Sampler is the only component of WVSS-II that has restrictions on air worthiness.   Because this requires a penetration of the aircraft skin, routine removal and inspection of the aircraft skin under the air sampler is generally required at intervals required by the aviation authority responsible for that STC.  This is typically intervals such as every 15,000 flight cycles. 

Occasionally the WVSS-II hoses may become clogged with insects of other debris, restricting airflow.  A simple maintenance procedure is defined to clear the hoses, and air carrier staff may occasionally be required to perform this cleaning.  In the event a sustained performance issue is noted by operational staff or WVSS-II users, the SEB can be removed and replaced with only a few hours of labor.

Other Costs Associated with Obtaining an STC

In addition to the Engineering labor associated with STC development, there are other costs associated with obtaining an STC.  This may include, but is not limited to, Travel and Lodging, document reproduction and delivery, and procurement of proprietary data from third party companies such as airframe manufacturers or suppliers, etc.   While as much collaboration as possible is done via telecommunications, travel may be necessary for meetings, initial installation for conformity support and inspections related to the STC process.
 

Acronym & Definitions

ACARS Aircraft Communication Addressing and Reporting System – Sends and receives information to and from the aircraft and the global networks from service providers Rockwell Collins or SITA.

ACMS Aircraft Condition Monitoring System – Sends reports, such as long-term trend data and aircraft/engine monitoring

AFM Aircraft Flight Manual

AMDAR Aircraft Meteorological Data Relay, a WMO program

AML Approved Model List

AMS100 Analyzer Management System software application from SpectraSensors

ARINC Originally Aeronautical Radio Incorporated, now simply ARINC

ASCII American Standard Code for Information Interchange

BOM  Australian Bureau of Meteorology

CAAC Civil Aviation Administration of China

CASA Civil Aviation Safety Authority of Australia

DFDAU Digital Flight Data Acquisition Unit

EMI ElectroMagnetic Interference

ETN Equivalent To New

EASA European Aviation Safety Agency

EWIS Electrical Wiring Interconnection Systems

FAA Federal Aviation Administration of the U.S.

ICA Instructions for Continued Airworthiness

JPL Jet Propulsion Laboratory, of NASA/Caltech

MDCRS Meteorological Data Collection and Reporting System, the US NWS AMDAR program

NASA National Aeronautics and Space Administration

NMHS National Meteorological and Hydrological Services

NWS U.S. National Weather Service

PSCP Project Specific Certification Plan

QMS Quality Management System

R&D Research and Development

SEB System Electronics Box

SSI SpectraSensors, Inc.

STC Supplemental Type Certification

TDL Tunable Diode Laser

TDLAS Tunable Diode Laser Absorption Spectroscopy

WMO World Meteorological Organization, a specialized agency of the United Nations

WVSS-II Water Vapor Sensing System-II