Maintenance and Certification
- Surge Damage!
- Engineering Test Pilots at Transport Canada: How Does Their Work Impact You?
- Carrying External Loads on Airplanes
- Continuing Airworthiness Division Reporting Systems
- Aircraft Certification Hosts 4th Delegates Conference in June 2006
- The System Safety Summer Briefing Kit is Now Available for Purchase!
Does an unexpected engine compressor surge event warrant the application of unscheduled engine maintenance?
Aircraft turbine engine operators are all familiar with the phenomenon of compressor surge (also frequently referred to as a compressor stall). A compressor surge is of special concern when engines with axial flow compressors are involved. Such engines may be equipped with up to 1 000 compressor blades, each of which can stall aerodynamically and start the onset of a compressor surge, with the possibility of various degrees of damage to an engine.
Although the compressor blades in an axial flow compressor act like airfoils and experience changes in airflow, pressure and velocity similar to those felt on an airplane wing, these blades do not physically change their position with respect to the air flowing past them. This means that the stalling of compressor blades is not identical to the stalling of an airplane wing, where the gradual increase of the angle between the chord of the wing and the on-coming air flow (the angle of attack) causes the wing to stall. Instead, compressor blade stalls should be thought of as something caused by changes in the effective angle of attack of the blades. The effective angle of attack depends on the velocity of the air entering the compressor and flowing past the blades, and the speed at which the blades are moving (compressor RPM). Changes in air velocity or compressor speed may cause the gradual onset of one or more blades stalling, with an eventual outcome of a compressor surge if enough blades stall. The compressor surge may cause such a disruption of airflow through the engine that the result will be mechanical damage to some of its components.
What's that noise?
When surging takes place, compressor airflow changes in pressure and velocity will cause anything from the most benign fluttering types of sounds, all the way to loud explosions. Only in severe cases of compressor surge does the pilot have the benefit of quantifying the effects of what is heard (by monitoring engine RPM and/or exhaust gas temperature). Most of the input is audible, perhaps accompanied by vibration, and does not lend itself to measurement. Since the source of the noise is caused by air slowing down, stopping, and even reversing in flow direction inside the engine, the noise severity is an indication of possible damage, keeping in mind that a worst-case scenario is complete engine failure. Since the causes of these air pressure and velocity changes can be attributed to factors such as fuel mismanagement, damaged or contaminated compressor blades, damaged turbine components and/or turbulent or disrupted airflow to the engine inlet, a number of engine maintenance-related factors are immediately introduced. Any components directly affected by the airflow through the engine, as well as those controlling fuel flow and over-pressurization of the compressor, might be suspect.
Will maintenance help?
Compressor surging is unpredictable, so the only kind of maintenance tasks that can adequately determine the effects of a compressor surge on the physical well-being of an engine, are unscheduled maintenance tasks. Since such tasks should be both applicable and effective, it must therefore be possible to do a comparison between the condition of an engine with no damage, and one that has sustained damage as a result of compressor surge effects. The inspection method would have to be applicable while the engine is installed and would generally include a visual inspection, most of which would entail the use of a borescope. The principal intent would be to inspect as many engine compressor and turbine blades as possible without disturbing the engine interior. Since the causes of some compressor surge events may be attributed to malfunctions of the engine's fuel control unit or of its bleed valves, these items may also qualify for some sort of inspection. Typically, the visual inspection of these items would verify their secure attachment to the engine and lack of evidence of damage or leakage. In some cases, operational or functional checks might prove beneficial.
What is applicable and effective?
Example of a fatique crack (arrow) on a compressor blade propagated from multiple origins on both convex and concave sides of the blade indicating cyclic loading in the reverse bending.
Why is no task prescribed?
Since the problem of compressor surge primarily affects engines with axial flow compressors, it may be assumed that a look in some of the maintenance manuals for such engines would lead to the discovery of surge-related maintenance tasks. A general review of the unscheduled inspection sections of these manuals reveals that no mention is made of compressor surge as an unscheduled event that needs to be addressed by the application of manufacturers' recommendations. There are good reasons for this.
The designers of modern turbine engines create products that will function efficiently over the full operational range of the aircraft in which the engine is to be installed. Some of the most innovative skills focus on the design of compressors and turbines that have to tolerate a wide range of air velocity, pressure and temperature conditions. Since all compressors are potentially plagued by the onset of blade stalling, supreme efforts are made to prevent such stalling from becoming a compressor surge. The result of all of this effort is the marketing of an engine that will tolerate some unintentional abuse, and will not have any significant compressor surge problem designed into it. This means that unscheduled maintenance tasks recommended in the maintenance manuals will primarily be restricted to those needed to rectify the effects of externally caused events, including such things as bird strikes, lightning strikes and exceeding engine-operating limits, etc. A review of aircraft flight manuals seems to support this logic as well, since the text devoted to engine malfunctions restricts itself to exceeding engine limits and engine failure (without specifying the many possible causes). Compressor surge events are therefore treated no differently than any other seldom-experienced event that may interfere with engine operation.
Is the status-quo acceptable?
In the past, there have been calls for an increase in the level of flight safety concern when there is evidence that compressor surge has contributed to the incidence of engine damage. These concerns typically have their origin in reports published by flight safety investigators, when it is determined that in-flight engine shut-downs were caused by compressor or turbine blade fatigue mechanisms that ended in blade failure. Such failures are potentially dangerous if the blade containment system on an engine suffering blade failure does not work as advertised. The result then, is possible damage to the aircraft and/or its occupants. Investigators therefore tend to address failure events by recommending the introduction of unscheduled maintenance tasks in the maintenance manuals.
Although such actions are well intentioned, it opens the door to adding a host of possibilities for treating other parts of the engine as potential sources for in-flight shutdowns. A more effective solution might be to make better use of the need for effective pilot reporting with respect to compressor surges. For instance, if it can be determined that the current frequency of compressor surge incidents on commercial aircraft warrants the addition of instructions in flight manuals to report such incidents, pilot reporting might prove advantageous. So far, it appears that even in the absence of such instructions, pilots are reporting surge events and some form of maintenance is being applied as a result. What is also evident is that some of the maintenance actions have not proven effective. In some cases, pilots reported several compressor surge incidents on the same engine, yet no effective maintenance action (such as disassembling the engine and inspecting the engine blades) was taken. Although a thorough borescope inspection of the accessible compressor and turbine blades may be done, followed by engine power assurance runs, fatigue-related defects would be difficult to find on these blades, and impossible to locate on the inaccessible ones. The problem, therefore, is one that involves the cooperation between pilots and maintenance workers in reporting any unusual engine events, and taking the most appropriate maintenance action in response to such reports.
The operator's maintenance program is the key...
Whenever flight manuals or maintenance manuals fail to address compressor surge events specifically, it is up to each operator to ensure that such events do not impact the safety of the operation. The effects of compressor surge problems identified by an operator as the result of unique operating conditions can be mitigated through the introduction of a combination of revised operating and reporting procedures and appropriate maintenance actions. The aircraft manufacturer should be consulted during the search for solutions to the problem, so that the benefits of lessons learned from the operation of the entire fleet can be factored into an operator's solution. Although this appears to place the burden for taking action entirely on the shoulders of the operator, it avoids the application of a "broad-brush" treatment of a problem currently recognized by the industry as one that occurs infrequently and generally has a minor impact on aviation safety. The focus, therefore, should be on ensuring that operators have a suitable system in place that formally addresses what actions to take in the event of a compressor surge. Such a system should include steps to be followed by pilots when reporting, as well as a clear explanation of the proper investigation and rectification procedures to be used by maintenance personnel. It must be recognized that the tendency to avoid engine removal and disassembly (overhaul) is always a strong factor working against recognizing the need to prescribe higher levels of engine maintenance. It is therefore of paramount importance to make the safe decision when it is necessary to do so, and forego the temptation to fly the aircraft one more time, in order to determine if the problem has been solved.
...but regulators have a role to play as well
To provide operators with some assistance, it will be necessary to ensure that manufacturers' recommendations for maintaining an aircraft include specific information addressing unexpected compressor surge events. The unscheduled maintenance section of the maintenance manual provides the opportunity to categorize these events along with such things as bird strike and lightning strike events. Under the umbrella of the regulations governing the need for adequate instructions for continued airworthiness, regulators can impose special conditions on aircraft manufacturers as part of the product certification activity. Such conditions should highlight the need for appropriate unscheduled tasks, directing the operator to effective troubleshooting procedures and clear recommendations to remove the engine from service, if on-wing maintenance fails to rectify the problem. The joint efforts by industry and regulators will thus ensure the enhancement of aviation safety.
During the take-off roll, the right in-board flight spoiler inadvertently deployed. The captain made a small lateral control input to counter the roll tendency, and the takeoff was continued with no further comment by either pilot. During the after-takeoff check, the first officer noticed on the flight control indicator that the right in-board spoiler was deployed. The captain had already trimmed the airplane to counter the deflection, but otherwise had not noticed the failure. A return to land was commenced and completed without further incident.
Who assesses an aircraft's handling qualities and decides if such a failure is acceptable? In aircraft certification, it is the job of the engineering test pilot.
Understanding the part that we play, and how it affects the operational line pilots who fly in the industry, requires a brief discussion of the certification standards. The Canadian Aviation Regulations (CARs) call up the certification design standards contained in the Airworthiness Manual chapters 523, 525, 527 and 529, which are the design requirements for small and large fixed-wing and rotary-wing aircraft. Each of the chapters address: structure; design and construction; powerplants; equipment; and operating limitations and information, but most relevant to the pilot community, is the flight subchapter, containing flying qualities and performance requirements. Of the approximately 400 paragraphs and sub-paragraphs in each chapter, there are some 140 items that require a test pilot qualitative evaluation. These items contain words such as "procedures consistently executed in service by crews of average skill," "time delays reasonably expected in service," "not require exceptional pilot skill, alertness or strength," "consistent results can be expected," "not cause undue difficulty in maintaining control," "safely controllable and manoeuvrable," "no excessive demands on the pilot when manoeuvring," "cannot be overstressed inadvertently," "stick forces within satisfactory limits," "suitable stability and control feel," "gradual, easily recognized, and easily controlled," "distinctive to the pilot," "prevent inadvertent stalling," and more. Although the test pilots are required to fly the aircraft to collect data for subsequent engineering analysis, it is the qualitative evaluations that perhaps have the most impact on line pilots.
RJ 900 water ingestion test
One example, for which there is no universally accepted solution, is the workload associated with flight management systems (FMS). Over the years, navigation has progressed from single-source aids, such as non-directional beacons (NDB) or VHF omni-directional ranges (VOR), to multi-sensor FMSs with inputs from ground- and space-based equipment. These FMSs reduce the pilot workload in terms of being able to navigate, but increase pilot workload in the management of the system. Some of the earlier, less sophisticated, systems were so cumbersome to input data that they were not certifiable, in our opinion. Even the systems that have been certified require the line pilots to be knowledgeable about the system and, perhaps more importantly, to have good cockpit discipline in terms of work sharing and standard operating procedures (SOP). So, even though we, the certification test pilot, might say something is certifiable, you, the operational pilot, have an essential part to play in using the equipment properly and safely. With new aircraft certification programs, it is routine to involve operational pilots with certification test pilots in a joint activity to better ensure operational suitability in the real world.
In addition to certification test flying, the Chief, Flight Test, Aircraft Certification, is responsible for approving aircraft flight manuals (AFM) and master minimum equipment lists (MMEL). Post-certification activities require each of the individuals in the Flight Test Division to be responsible for several AFMs and associated MMELs.
AFMs contain the limitations, emergency procedures, normal and abnormal procedures, and aircraft performance. AFMs can contain both approved and unapproved data, which is clearly identified as such, and provides the procedures to be followed in the day-to-day operation of the aircraft and its equipment. In some cases, particularly for large aircraft, an associated operating manual (not a certification document) is provided by the manufacturer, and contains more detail to assist each operator in developing SOPs. Although not mandatory, operators for the most part adhere closely to manufacturer-recommended procedures for obvious reasons; that is, the manufacturer is most knowledgeable about the aircraft. Notwithstanding this flexibility, the AFM limitations are considered mandatory. Finally, the AFM is a certification document and the test pilots, both regulatory and manufacturer, play a key role in its evolution throughout the certification program.
The individuals in the Flight Test Division (test pilots and flight test engineers) are responsible for chairing MMEL Review Groups that have regulatory, operational and manufacturer representation. These groups decide what equipment may be inoperative for flight dispatch. This relief is for a defined, short period of time with associated provisos. The relief is based on operational and certification considerations to include redundancy, next failure assessment, and qualitative (test pilot) and quantitative (engineering) evaluation. All this requires extensive communication with manufacturers and operators to ensure safety, and recognize the need to be able to fly with inoperative equipment.
I have just touched the surface of what we do from day to day. The variety in terms of flying different types of aircraft, travel, technical understanding, and associated office administration (did I say that?) makes this a tremendously interesting and challenging job. We like to think we make a valuable contribution, but are only one part of the "system" which makes flying safe.
The need for a considered and cautious approach to carrying external loads should be self-evident. As early as 1935, the National Research Council of Canada studied the practice of carrying canoes on float-equipped aircraft. Over the years, preferred methods of attachment, location and orientation of external loads have been established. For example, square back aluminium boats should be mounted with the stern facing forward to reduce adverse wake effects over the airplane tail. In addition, many canoe/boat/airplane combinations have been shown by experience to be airworthy, while other combinations are not considered safe.
Float plane carrying an external load
Unfortunately, accidents and incidents continue to occur with airplanes carrying external loads. In October 2003, a fatal loss-of-control accident involving a Piper PA-18-150 occurred while carrying moose antlers attached to the aircraft floats. The Transportation Safety Board report (TSB Report A03W0210) for the PA-18-150 accident cites more than 17 accidents that have occurred since 1976, involving external load operations with airplanes. Nine of these accidents involved fatalities.
Although many operators have obtained formal design approval for their external load installations, the current regulations are somewhat ambiguous on this subject, which has led to inconsistent interpretation and application of the rules related to the carriage of external loads.
Transport Canada is taking steps to amend the regulations and guidance material to clearly mandate that operators wishing to carry external loads must first get formal Transport Canada design approval for the installation. This requirement would apply to all external load operations for airplanes that are considered major design changes. Examples of external loads that are considered major design changes would include canoes and boats being carried on float-equipped aircraft.
- Means of securing the load to the aircraft. The means should be safe and repeatable.
- Location of external load. The load should not interfere with the propeller, wing lift struts, emergency egress for the pilot and passengers, or pitot-static ports. The location of the load should not permit the retention of water spray.
- Flight characteristics and performance. Usually, a flight test is required to verify adequate stability and control, engine cooling, climb performance and the absence of wake effects from the external load on the airplane empennage.
- Provision of operating and maintenance information applicable to the external load operation. This would normally include load attachment information as well as operating limitations and procedures. Often external load operations include a reduction in maximum take-off weight to cater for the reduced aircraft performance as a result of the aerodynamic drag of the loads.
Transport Canada records indicate that more than 150 different approvals for external loads have been issued over the years. Transport Canada Regional Aircraft Certification offices maintain a listing of all previously issued approvals. Many of the existing design approvals are available for purchase or use by airplane operators.
Applications for new design approvals of an external load should be made to the Transport Canada Aircraft Certification office located in the operator's region. Consult the Transport Canada Web site for contact details (www.tc.gc.ca/air/offices.htm). Operators are encouraged to utilize the services of Transport Canada Aircraft Certification delegates, such as design approval representatives (DAR) and design approval organizations (DAO), when seeking design approvals.
The Continuing Airworthiness Division of the Aircraft Certification Branch of Transport Canada Civil Aviation oversees the continuing airworthiness (CAW) of approximately 30000 Canadian-registered civil aircraft, as well as countless Canadian-designed and manufactured aeronautical products operated worldwide.
Successful execution of the regulatory component of the CAW activity is highly dependent on the efficient management and ready access by Transport Canada personnel to vast amounts of aircraft-specific data, documents, reports, and other information.
Two of the most significant and enduring of the early "legacy" systems developed by the Continuing Airworthiness Division to support their activities are the Service Difficulty Reporting System (SDRS) and the Computerized Airworthiness Information System (CAIS); both having been implemented in the late 1980s.
These two systems have been subject to capital investments in the past years in order to alleviate the workload to both external and internal stakeholders by allowing for enhanced reporting using state-of-the-art technology.
Web Service Difficulty Reporting System (WSDRS)
SDRS, which was the first to be converted to an enhanced Web-based application, facilitates the collection and retrieval of service problems encountered in the field. The information collected provides data to support the investigation and the development of corrective actions, where necessary.
Canadian Aviation Regulation (CAR) 591 requires that air operators, aircraft maintenance organizations (AMO), type certificate holders (including special type certificate holders), manufacturers, flight training units (FTU), distributors, and CAR 604 private operators submit service difficulty reports (SDR). Aircraft maintenance engineers (AME) working on private aircraft, or any small privately-operated aircraft, are also encouraged to submit SDRs.
Transport Canada receives approximately 2 200 SDRs annually. The Transport Canada WSDRS was developed as a result of requests from the Canadian aviation industry for a Web-based, fast, convenient and confidential SDRS.
Registered users can utilize this site to: submit SDRs as required by the CARs; query the SDR database; track and store submitted SDRs; update previously submitted SDRs; and check for Transport Canada action (status updates) on Canadian SDRs.
As of 2005, there are approximately 1 600 registered users of the WSDRS, representing 95 percent of large organizations (greater than 15 employees). Although the hardcopy form 24-0038 is still available, WSDRS has largely replaced its use for the reporting of service difficulties by Canadian industry.
Non-registered visitors to this site can search the SDR database using the "Quick Queries" buttons found on the side menu of the WSDRS homepage.
For more information on the WSDRS, visit the following Web site:
Web Service Difficulty Reporting System (WSDRS).
Continuing Airworthiness Web Information System (CAWIS)
CAIS performed a number of essential functions, including: recording airworthiness and owner information for approximately 30000 Canadian-registered aircraft; collecting and disseminating aircraft utilization data (hours flown) through the CAR-enabled Annual Airworthiness Information Report (AAIR); storing, indexing and facilitating public on-line access to all 40000 airworthiness directives (AD); facilitating selective distribution of corrective action notifications (such as ADs) to the affected parties; and other miscellaneous functions.
Requests from both registered aircraft owners and Transport Canada personnel for a Web-based airworthiness information system have resulted in the development of the CAWIS Web site.
CAWIS is primarily used by registered owners, operators, maintainers and manufacturers of Canadian-registered aeronautical products or products for which Canada is the country of type design responsibility, as well as Transport Canada personnel.
Registered aircraft owners log on to CAWIS using an AAIR access code that is indicated directly on the top right corner of the AAIR form, which is mailed to them by Transport Canada. Registered users can also utilize this site to query the AD database (for both foreign and domestic ADs) and review data pertaining to their own aircraft.
Visitors of CAWIS can search the AD database by selecting the Airworthiness Directives link located on the side menu (just below the login button) of the main page.
For more information on CAWIS, visit the following Web site:
Continuing Airworthiness Web Information System (CAWIS).
The Aircraft Certification Branch is again hosting a delegates conference. The 2006 conference will be held at the Ottawa Congress Centre, June 27–29, 2006. The previous conference, held in 2003, attracted over 500 participants, and a similar turnout is predicted. All Aircraft Certification Delegates are invited to attend. Registration to date has been very positive; the conference is over 75% sold out.
The theme for the conference is "Aircraft Safety Through Delegation." In addition to the plenary session, specialist streams have been set up to cover the areas of flight test; avionics/electrical software; aircraft structures; powerplants and emissions; fuel and hydro mechanical control systems; and occupant safety and environmental systems. Program information can be found on the Web site indicated below.
The conference program has been developed by an organizing committee made up of representatives from Industry and Transport Canada, and has been designed to appeal to all delegates.
The objective of the conference is twofold. The first and foremost objective is to educate delegates and Transport Canada personnel on regulatory developments, policy initiatives, and new technology. The second objective is to strengthen the combined Industry and Transport Canada Aircraft Certification Team, which is essential to meet the challenges facing the Industry and to maintain Canada's leading role in aviation.
We encourage you to take advantage of this opportunity to strengthen your working relationship with the combined Transport Canada/ Delegates Team.
Invitations to the conference have been sent to all delegates; if you did not receive one, please register. This can be done electronically,
at http://wwwapps.tc.gc.ca/saf-sec-sur/2/dc-cd/, or by contacting Mr. G. Adams at 613 941-6257, or e-mail ADAMSGL@tc.gc.ca. The organizing committee will confirm your registration by separate correspondence. The organizing committee finalized the conference program in December 2005, and will publish it on the Web site in early 2006.
This six CD-ROM collection contains various promotional products produced by System Safety headquarters and regional offices. This package was originally designed to provide the regional System Safety Specialists with a central bank of materials for the regional safety briefings. However, this collection could well serve Industry in setting up their own safety briefings, in the same way we announced the availability of the System Safety Winter Briefing Kit in ASL 3/2005. The cost for the summer briefing kit is $25, and its contents can be viewed at: http://www.tc.gc.ca/eng/civilaviation/publications/tp14112-menu-316.htm.
- CD 1: Runway Incursion Prevention Tools; this CD has the video "Danger on the Runway," the six runway incursion prevention posters, past newsletters articles on runway incursions and more.
- CDs 5 and 6: Weather to Fly CDs; these two CDs contain 26 two-minute video vignettes aimed at general aviation pilots and the general public. The aim of these vignettes is to promote safe flying and how weather affects flight conditions and is a factor in every flight.
The System Safety Summer Briefing Kit (TP 14112E) can be purchased from the new Transport Canada Transact Web site at www.tc.gc.ca/transact, or by calling the Civil Aviation Communications Centre
at 1 800 305-2059.
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