This article focuses on the multiple medium flocking bird test and also presents a memoir of Robert S. Mazzawy, Principal Engineer, Trebor Systems, LLC. Multiple requirements for demonstration testing of bird ingestion tolerance at takeoff power for new commercial transport engines have also been discussed in the article. For the engine test set up, the engine is configured only with production level instrumentation necessary for the engine control – rotor speeds, fuel flow, burner pressure, exhaust gas temperature, etc. The dynamics of the fan/booster decelerating more slowly than the core compressor created a situation in which the flow capacity of the core compressor drops below the level necessary for stable operation of the booster compressor. In recognition of these dynamics, engines are designed with a surge recovery bleed between the booster and core compressor to compensate for this flow mismatch. The key to resolving this dilemma is to have the engine control recognize that additional surges after normal closure of the surge recovery bleed are indicative of damage to the fan and loss of flow pumping.
FAA certification of new commercial transport engines requires demonstration testing of bird ingestion tolerance at takeoff power under FAR §33.76. There are multiple requirements that include:
One Large Bird (e.g. Canada goose) weighing from 4 to 8 lbs depending on engine size.
Engine must not catch fire and be able to be safely shut down after 15 seconds with no throttle movement.
Multiple Medium Flocking Birds (e.g. gulls) weighing 1.5 to 2.5 lbs.
Engine must continue to deliver at least 75% of normal takeoff thrust for 2 minutes with no throttle movement followed by a ~20 minute run-on period at various power levels consistent with aircraft return to airport and landing.
One Large Flocking Bird (e.g. snow goose) weighing 4 to 5.5 lbs.
Engine must continue to deliver at least 50% of normal takeoff thrust for 1 minute followed by a ~20 minute run-on period.
The single large bird test targets the most critical area of the fan blade. The single large flocking bird is targeted at 50% fan blade span. The multiple medium flocking birds are targeted at critical areas of the fan plus at least one bird must enter the engine core. This article focuses on the multiple medium flocking bird test.
The Big Bang
I will begin this article by relating my practice as an engineer at Pratt & Whitney for “witnessing” the medium (flocking) bird ingestion certification test for a new engine. Rather than joining the crowd of test and project engineers and FAA officials in the control room, I would always stand in the parking area just outside of the inlet to the test stand. At this location, one could hear the engine roar into life and accelerate to full takeoff power, and then after a few minutes, the “big bang” characteristic of the engine surge associated with birds being ingested into the engine core. This event is clearly illustrated in the first figure and the bang is the shock wave associated with engine flow reversal as evidenced by flame out the inlet. The “big bang” would be followed immediately by one of two possibilities. The one everyone hoped for was a change in the engine’s noise characteristic and amplitude signifying the damage to fan blades and importantly a rapid surge recovery. The other possibility was the quieter sound of a rapid engine deceleration and shut down, indicative of a test failure. As mentioned earlier, failure here means the engine is unable to deliver at least 75% of normal thrust for the 2 minutes without throttle movement followed by the ~20 minute run-on period after the ingestion event. Meeting these requirements demonstrates the ability of the engine to complete the takeoff without pilot action after which the pilot can make a go around to land the aircraft.
Now what caught my interest early on was the fact that in most cases when an engine “failed” the test, it could be restarted and run successfully for longer than the post-ingestion requirement with more than the necessary thrust. Subsequent engine tear-down and inspection typically showed no significant core damage and tolerable fan blade damage. To unravel this mystery, let’s review the sequence of events leading up to the test and the milliseconds immediately after the “big bang”.
Engine Test Set Up
For this test, the engine is configured only with production level instrumentation necessary for the engine control – rotor speeds, fuel flow, burner pressure, exhaust gas temperature, etc. The special test equipment consists of the cannons that fire the euthanized birds and the high speed cameras and special lighting. As can be seen in the following figure, the fan blades are painted white and numbered to allow the confirmation that the flock of birds are spread across the fan and meet the pre-agreed targets in terms of blade span. These targets are selected to assure that all critical areas of the fan blade are hit by at least one bird and that one or more birds enter the engine core.
After the Big Bang
An engine surge typically requires only about 150 milliseconds for the flow to reverse, the burner to depressurize and then re-establish flow and burner pressure. However, during a bird strike the bird becomes “fluidized” and takes a similar time to traverse the engine compression system length and arrive at the combustor. Here the fluid becomes gasified, necessarily expands due to greatly reduced density and back pressures the compressor. This may delay the surge recovery and can even precipitate a second surge event. Now during the short time span of the surge, the engine rotor speeds fall sharply due to loss of pressure required to drive the turbines. The main core compressor rotates independently of the fan and booster compressor and decelerates more quickly due to its lower moment of inertia. In addition to the high moment of inertia of the fan, any damage to the fan reduces its ability to pump air. This reduced air pumping load further reduces the rate of deceleration of the fan/booster. I was able to use the high speed camera records and numbered fan blades to verify that more fan damage resulted in reduced fan deceleration during a bird strike induced surge.
The dynamics of the fan/booster decelerating more slowly than the core compressor created a situation in which the flow capacity of the core compressor drops below the level necessary for stable operation of the booster compressor. In recognition of these dynamics, engines are designed with a surge recovery bleed between the booster and core compressor to compensate for this flow mismatch. The surge recovery bleed timing is rapid enough to stabilize the engine within one or two surges. All of this takes place so rapidly that human senses only detect what seems like a single “big bang”.
Now all should be well once the surge bleed opens and restores stable operation and subsequently closes as dictated by the engine control surge recovery logic. Why then does the engine fail to operate in some bird strike tests? The key differential is the amount of damage to the fan blades and resultant loss of flow pumping load. With similar power from the turbine driving the fan/booster, this leads to higher than normal fan/booster rotor speed relative to the core compressor. If this mismatch of rotor speeds is great enough, the booster cannot operate stably and repetitive surging becomes inevitable when the normal closure of the surge recovery bleed takes place.
The key to resolving this dilemma is to have the engine control recognize that additional surges after normal closure of the surge recovery bleed is indicative of damage to the fan and loss of flow pumping. Once the control recognizes this sequence, it can limit the surge recovery bleed to only a partial closure. Doing so allows the engine to remain stable and deliver the necessary thrust to enable the pilot to safely land the aircraft. I can happily report that once this feature became a standard control element in Pratt & Whitney engines, the uncertainty of the “big bang” aftermath became a part of past history. Were someone to stand near the test stand inlet today, he or she would hear the intended sequence of “big bang” followed by extended engine operation until its intentional controlled shutdown at the conclusion of the test.