Tuesday, August 1, 2006
Aircraft turbine engine operators are familiar with compressor surge, also frequently called compressor stall. The phenomenon is of special concern when it involves engines with axial flow compressors, which may be equipped with up to 1,000 compressor blades. Each blade can stall aerodynamically and start a compressor surge, causing various degrees of damage to an engine.
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. However, compressor blades do not change their position relative to the air flowing past them. Their stalling, therefore, is not identical to the stalling of an airplane wing, where the gradual increase of the angle between the wing's chord and the on-coming air flow (the angle of attack) causes the stall.
Instead, compressor blade stalls are caused by changes in the blade's effective angle of attack. It depends on the velocity of the air entering the compressor and flowing past the blades, as well as 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. If enough blades stall, you eventual have a compressor surge, and this may cause such a disruption of airflow through the engine that it may damage 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 sounds to loud explosions. Only in severe cases of compressor surge does the pilot have the benefit of quantifying the effects of what he has heard, by monitoring engine RPM and/or exhaust gas temperature.
Most input from the surge 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.
Compressor surging is unpredictable, so the only maintenance tasks that can adequately determine its effects on the well being of an engine are unscheduled maintenance tasks. To assure the tasks are both applicable and effective, a comparison should be made between an engine with no damage and one with damage from a compressor surge.
The inspection method would have to be applicable while the engine is installed and would generally include a visual inspection, mostly using a borescope. The principal intent would be to inspect as many engine compressor and turbine blades as possible without disturbing the engine interior.
Types of Inspection
In all cases of compressor surging, any maintenance action would have to be triggered by a report that an unacceptable level of surging was detected.
In any case, a number of inspection options are available:
- First is the general visual inspection, which looks for obvious damage to the accessible parts of the aircraft and engine.
- Next would be the detailed inspection, which may particularly benefit suspect parts of the engine. It looks for less obvious damage and may involve the use of special tooling, such as a borescope, designed to provide access to the engine interior without extensive engine teardown. (Unfortunately, access provisions for borescopes [long slender bundles of light-carrying fibers] are limited, so only a small percentage of compressor or turbine blades can be inspected using this method.)
- The highest level of inspection, the special detailed inspection, typically requires extensive preparation or special inspection techniques and tooling.
No Mention in Manuals
Since the problem of compressor surge primarily affects engines with axial flow compressors, one might assume that the maintenance manuals for such engines would include surge-related maintenance tasks.
However, a review of the unscheduled inspection sections in these manuals reveals no mention of compressor surge as an unscheduled event that needs to be addressed as part of the manufacturers' recommendations.
There are reasons for this. Since all compressors are potentially subject to blade stalling, supreme efforts are made in design to prevent the stalling from becoming a compressor surge. The result is the marketing of engines that will tolerate some unintentional abuse and not have any significant compressor surge problem designed into it.
This means the unscheduled maintenance tasks recommended in the maintenance manuals will primarily be restricted to those rectifying the effects of externally caused events, including bird strikes, lightning strikes, engine-operating exceedances, etc.
Is This Acceptable?
Pilots who report surge events could change the procedures outlined in maintenance manuals, however. If it can be determined that the frequency of compressor surge incidents on commercial aircraft warrants additional instructions in flight manuals to report such incidents, pilot input may cause such change.
So far, even without such instructions, pilots are reporting surge events, and some form of maintenance is being applied as a result.
Evidence also exists that some of the maintenance actions have not proven effective. For example, pilots have 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 find on the inaccessible blades.
The problem, therefore, involves the cooperation between pilots and maintenance workers in reporting any unusual engine events, and taking the most appropriate maintenance action in response.
Whenever flight manuals or maintenance manuals fail to address compressor surges specifically, it is up to the operator to ensure that such events do not impact operational safety.
The effects of compressor surge problems resulting from unique operating conditions can be mitigated by introducing revised operating and reporting procedures, as well as appropriate maintenance actions. The aircraft manufacturer should be consulted during the search for solutions, to gain input from other operators of the same engine.
True, this process places more work on the operator. But it is better than the "broad-brush" treatment of a problem that currently is viewed as occurring infrequently and causing only minor impact on aviation safety.
The tendency to avoid engine removal and disassembly (overhaul) always works against recognizing the need to prescribe higher levels of engine maintenance. The challenge, therefore, is to forego the temptation to fly the aircraft one more time, in order to determine if the problem has been solved.
To assist operators, the manufacturers' maintenance recommendations should include information specific to unexpected compressor surge events. The manual's unscheduled maintenance section provides the opportunity to categorize these events, along with bird strikes, lightning strikes, and such.
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 would highlight the need for appropriate unscheduled tasks. It would direct the operator with troubleshooting procedures and clear recommendations to remove the engine from service, if on-wing maintenance fails to resolve the issue. The joint efforts by industry and regulators will thus ensure the enhancement of aviation safety.
John Tasseron is a safety inspector with Transport Canada (TC), reviewing Instructions for Continued Airworthiness as part of aerospace product certification. A more extensive version of this article first appeared in the TC Aviation Safety Letter.