Turbojet Engine Failure Prevention
THREE Boeing airliners recently suffered in-flight uncontained engine failures in as many days, the last weekend of February 2021. A United Boeing 777 out of Denver AND a Delta Boeing 757 out of Atlanta AND a Longtail Boeing 747 out of Holland. Engine parts were scattered across the countryside, damaging homes and vehicles, luckily only injuring a few people. All aircraft suffered minor damage, but if any had a fuel tank ripped open, they would have most likely caught fire and crashed, killing all on board; as has happened to many other aircraft.
This has prompted me to explain why I tried, years ago, to develop an in-flight turbojet engine failure warning system and how it was canceled. I only wish I had tried harder.
Hundreds have been damaged or destroyed and thousands of lives lost in the past to uncontained engine failures; which might have been saved. It is only a matter of time before another uncontained engine failure throws blades or buckets into cabins, fatally wounding passengers, or rips apart fuel tanks or controls, crashing the aircraft.
The first passenger jetliner, DH-106 Comet, had engines inside wings for aerodynamic efficiency, but also wrapped in armor to contain engine failures. Later passenger jets had their engines mounted outside on “fuse-able bolts”, so a failing engine could be ejected before it caused more damage. However, Concord had engines in its wings.
Crack detection inspections of compressor blades every 3,000 hours has apparently not reduced the failure rate, which may progress from a crack to stress rupture in a few hundred hours or less. Fortunately, cracked blades don't rupture immediately, as some have maintained, which would also make even 1,000 hour inspections worthless.
Foreign Object Damage or Hydrogen Embrittlement Cracks?
In 1958, USAF/OCAMA initiated a study of Foreign Object Damage (FOD) limits on compressor blades as related to engine failures, since engine failures were becoming relatively frequent and the manufacture's FOD limits were suspect. I and others were tasked by the USAF to determine the limits of FOD to be allowed during overhaul.
However, I concluded the cause of most failures was Hydrogen Embrittlement cracking on the convex mid span surface; where engineers allowed the greatest FOD damage. Failures also seemed to be associated with vibration during compressor stall. Here the flexing of blades could result in a migration of hydrogen atoms into grain boundaries, forming hydrogen molecules. In time, enough of these molecules might collect and split crystal grain boundaries, leading to cracking and eventual stress rupture.
In 1959, I wrote an investigation summary and suggested the use of a device to detect impending failures; sending it to USAF/WPADC. I insisted it might be better to develop an in-flight detection device to warn of any decreasing clearance in the gap between the inside of an engine's housing and a blade's tip; the first sign of impending failure. I also insisted that vacuuming runways to eliminate the ingestion of foreign materials and then blaming FOD after engine failures might not be realistic or cost effective.
There was no reply from USAF/WPADC, but the manager of GE engine development wrote to my boss, possibly complaining about my efforts and noted, “We have NOT conducted any vibratory stress studies on the subject blades”, citing the lack of slip rings. However, I found slip rings were available from a supplier in Chicago which could transmit strain gauge readings on blades in an operating engine to recorders.
We determined the maximum stress was in an area of the blades where GE engineers allowed the maximum amount of FOD to be approved during engine overhaul and the lowest stress was on leading edges, where GE restricted the least amount of FOD.
1960, my work was recognized in a USAF/OCAMA report on FOD limits, but no mention was made of any suggestion or need to detect impending engine failures. So I resigned my position to develop my own version. This involved running an import car repair garage along with spending long hours designing test equipment and trying to obtain information, used blades and an operational engine to run with cracked blades.
Fortunately, a scrap metals dealer allowed me to sort thru his piles of rejected blades; from USAF/OCAMA. I found many cracked, using dye penetrate, with evidence of repeated service periods. These appeared as “tree rings” when broken apart. Clearly, there was no danger of sudden failure when the crack started and blade separated.
Oden Research Corp
1963, I formed a stock company to develop my earlier USAF/OCAMA suggestion, which was progressing nicely. I then submitted an application for a federal grant to pay for a destructive test using an operating engine. I hoped to develop my system as a means of preventing in-flight contained or uncontained engine failures, even bearing failures.
June 20, 1965, Pan Am Flight 843, 707-321B, on take-off from San Francisco, the #4 engine or right outboard had an uncontained failure. The engine tore loose, went right and ripped off 25 feet of the right wing. (The pilot was still able to land safely!)
This event convinced Larry Booda, Aviation Week Editor, to arrange my appearance before the Senate Subcommittee on Aviation Safety; regarding my failure warning system. There a civilian engineer testified my device might work in theory, but was not useful because blade failure happens too quickly to be detected. (Yet cracked blades with evidence of repeated service periods or safe flight hours were often found during engine overhaul at USFA/OCAMA; later found by me in a metal dealer's salvage yard.)
A military engineer testified that USAF policy was “Fly to Fail” and depend on ejection seats to save the crew or for the engines to ejected "safely" on multi-engine aircraft. He claimed the addition of more gauges or warning lights would be “Distracting”.
After the hearing was over, a young man came to me and said, "We are already using a version of your device in our test cells to prevent damage from uncontained failures during development testing.” (There is a record of several test cells, within armored buildings, being destroyed by uncontained engine failures.)
I asked, and still ask, “Why not use your blade tip gap clearance detection in-flight?” He walked away without any reply. Later, the committee rejected my application.
On my return home, the scrap metals dealer, from whom I obtained cracked blades rejected by USAF/OCAMA and who promised me a free operational engine for testing my blade tip gap clearance detection device, asked to meet me at the local airport. Then he asked to walk out to my airplane where we might not be overheard. Then he said, “You really pissed off someone important. I can't allow you to come around again.” That ended my efforts to develop an impending engine failure warning system.
I eventually became a new car dealer and local Civil Air Patrol Squadron Commander; with 26 years of service. I also have an active interest in the commercial uses of Hydrogen and my company has been drilling for same as a fossil fuel alternative.
ROTOR BLADE MONITORTM Turbojet Engine Failure Warning System
One model with two modes were considered. One mode for use in maintenance intervals. The other mode as an in-flight warning of blade tip gap reduction or blade elongation, which has an unknown time period before stress rupture and engine failure.
However, blades and buckets apparently do not fail immediately when cracks and elongation becomes evident, whether said to have been caused by FOD or Hydrogen Embrittlement. 3,000 hour inspections appear routine, with 1,000 hour inspections being considered.
Maintenance Mode consisted of a dual electrode spark plug fitted to engine inspection ports that allow the inspections of individual rows of compressor blades or turbine buckets.
A central electrode, projecting “X” inches inside the engine housing, is centered inside a circular electrode, mounted flush with the engine housing. An electronic spark is passed between the electrodes, with a gap of “X” inches, while blades or buckets have a tip gap clearance inside the engine of “3X” inches.
When the blades/buckets start cracking, centrifugal force moves the tips outward. It is worth noting that blades/buckets function as in-line springs, with their crystals able to flex ever so slightly. How much they extend at 80% of max RPM is the basis of my warning system. Of course this “3X” blade tip gap inside the engine dimension varies with different engines.
The frequency of the spark could be varied to correspond to near engine RPM, allowing each blade or bucket tip gap to be examined for its extension in apparent slow motion.
When a cracking blade/bucket extends more than “X” distance, it will be closer to the central electrode than the “X” distance to the circular electrode. The spark will then travel to the tip of the elongated blade/bucket, not to the second electrode. The absence of spark to the second electrode would be noted. With the addition of a timing feature, any elongated blade/bucket could be located exactly where it may be found on the hub.
I had envisioned an aircraft taxiing to a maintenance hanger, an analysis consul plugged into a fitting on the engine and all blades and buckets, even bearings, checked for elongation or wear in a relatively short period of time. Then, if a blade or bucket or bearing was found to be elongated or worn, the engine could be promptly exchanged and sent to overhaul, and the aircraft returned to service; a more cost effective system than before.
In-Flight Mode consists of the same dual electrode spark plug with the main electrode still projecting into the engine housing “X” distance; now acting as a rub point. When a blade or bucket tip in any row of any engine extends more than ”2X” distance, it will touch the rub point. This grounds a circuit, sending an individual engine's signal to a warning light in the cockpit. The pilot then takes action as is useful. Only one light needed per engine.
Hundreds of cracked and elongated blades that have rubbed the inside of their engine housings have been found during overhaul, indicating hours of operation after elongation. So the pilot might take note of an engine with a blade or bucket elongating, reduce power on the identified engine, try to avoid compressor stall, and make a safe landing.
The in-flight warning system was in my suggestion to USAF/WPADC in 1959. Whereas the Maintenance Mode Model was what I was trying to develop before I testified in the Senate aviation safety committee hearing. Then I was effectively prevented from doing more. My hope is that someone might be able to convince an engine manufacturer to install what I was told was a similar warning system into engines used in-flight and save lives.
THREE Boeing airliners recently suffered in-flight uncontained engine failures in as many days, the last weekend of February 2021. A United Boeing 777 out of Denver AND a Delta Boeing 757 out of Atlanta AND a Longtail Boeing 747 out of Holland. Engine parts were scattered across the countryside, damaging homes and vehicles, luckily only injuring a few people. All aircraft suffered minor damage, but if any had a fuel tank ripped open, they would have most likely caught fire and crashed, killing all on board; as has happened to many other aircraft.
This has prompted me to explain why I tried, years ago, to develop an in-flight turbojet engine failure warning system and how it was canceled. I only wish I had tried harder.
Hundreds have been damaged or destroyed and thousands of lives lost in the past to uncontained engine failures; which might have been saved. It is only a matter of time before another uncontained engine failure throws blades or buckets into cabins, fatally wounding passengers, or rips apart fuel tanks or controls, crashing the aircraft.
The first passenger jetliner, DH-106 Comet, had engines inside wings for aerodynamic efficiency, but also wrapped in armor to contain engine failures. Later passenger jets had their engines mounted outside on “fuse-able bolts”, so a failing engine could be ejected before it caused more damage. However, Concord had engines in its wings.
Crack detection inspections of compressor blades every 3,000 hours has apparently not reduced the failure rate, which may progress from a crack to stress rupture in a few hundred hours or less. Fortunately, cracked blades don't rupture immediately, as some have maintained, which would also make even 1,000 hour inspections worthless.
Foreign Object Damage or Hydrogen Embrittlement Cracks?
In 1958, USAF/OCAMA initiated a study of Foreign Object Damage (FOD) limits on compressor blades as related to engine failures, since engine failures were becoming relatively frequent and the manufacture's FOD limits were suspect. I and others were tasked by the USAF to determine the limits of FOD to be allowed during overhaul.
However, I concluded the cause of most failures was Hydrogen Embrittlement cracking on the convex mid span surface; where engineers allowed the greatest FOD damage. Failures also seemed to be associated with vibration during compressor stall. Here the flexing of blades could result in a migration of hydrogen atoms into grain boundaries, forming hydrogen molecules. In time, enough of these molecules might collect and split crystal grain boundaries, leading to cracking and eventual stress rupture.
In 1959, I wrote an investigation summary and suggested the use of a device to detect impending failures; sending it to USAF/WPADC. I insisted it might be better to develop an in-flight detection device to warn of any decreasing clearance in the gap between the inside of an engine's housing and a blade's tip; the first sign of impending failure. I also insisted that vacuuming runways to eliminate the ingestion of foreign materials and then blaming FOD after engine failures might not be realistic or cost effective.
There was no reply from USAF/WPADC, but the manager of GE engine development wrote to my boss, possibly complaining about my efforts and noted, “We have NOT conducted any vibratory stress studies on the subject blades”, citing the lack of slip rings. However, I found slip rings were available from a supplier in Chicago which could transmit strain gauge readings on blades in an operating engine to recorders.
We determined the maximum stress was in an area of the blades where GE engineers allowed the maximum amount of FOD to be approved during engine overhaul and the lowest stress was on leading edges, where GE restricted the least amount of FOD.
1960, my work was recognized in a USAF/OCAMA report on FOD limits, but no mention was made of any suggestion or need to detect impending engine failures. So I resigned my position to develop my own version. This involved running an import car repair garage along with spending long hours designing test equipment and trying to obtain information, used blades and an operational engine to run with cracked blades.
Fortunately, a scrap metals dealer allowed me to sort thru his piles of rejected blades; from USAF/OCAMA. I found many cracked, using dye penetrate, with evidence of repeated service periods. These appeared as “tree rings” when broken apart. Clearly, there was no danger of sudden failure when the crack started and blade separated.
Oden Research Corp
1963, I formed a stock company to develop my earlier USAF/OCAMA suggestion, which was progressing nicely. I then submitted an application for a federal grant to pay for a destructive test using an operating engine. I hoped to develop my system as a means of preventing in-flight contained or uncontained engine failures, even bearing failures.
June 20, 1965, Pan Am Flight 843, 707-321B, on take-off from San Francisco, the #4 engine or right outboard had an uncontained failure. The engine tore loose, went right and ripped off 25 feet of the right wing. (The pilot was still able to land safely!)
This event convinced Larry Booda, Aviation Week Editor, to arrange my appearance before the Senate Subcommittee on Aviation Safety; regarding my failure warning system. There a civilian engineer testified my device might work in theory, but was not useful because blade failure happens too quickly to be detected. (Yet cracked blades with evidence of repeated service periods or safe flight hours were often found during engine overhaul at USFA/OCAMA; later found by me in a metal dealer's salvage yard.)
A military engineer testified that USAF policy was “Fly to Fail” and depend on ejection seats to save the crew or for the engines to ejected "safely" on multi-engine aircraft. He claimed the addition of more gauges or warning lights would be “Distracting”.
After the hearing was over, a young man came to me and said, "We are already using a version of your device in our test cells to prevent damage from uncontained failures during development testing.” (There is a record of several test cells, within armored buildings, being destroyed by uncontained engine failures.)
I asked, and still ask, “Why not use your blade tip gap clearance detection in-flight?” He walked away without any reply. Later, the committee rejected my application.
On my return home, the scrap metals dealer, from whom I obtained cracked blades rejected by USAF/OCAMA and who promised me a free operational engine for testing my blade tip gap clearance detection device, asked to meet me at the local airport. Then he asked to walk out to my airplane where we might not be overheard. Then he said, “You really pissed off someone important. I can't allow you to come around again.” That ended my efforts to develop an impending engine failure warning system.
I eventually became a new car dealer and local Civil Air Patrol Squadron Commander; with 26 years of service. I also have an active interest in the commercial uses of Hydrogen and my company has been drilling for same as a fossil fuel alternative.
ROTOR BLADE MONITORTM Turbojet Engine Failure Warning System
One model with two modes were considered. One mode for use in maintenance intervals. The other mode as an in-flight warning of blade tip gap reduction or blade elongation, which has an unknown time period before stress rupture and engine failure.
However, blades and buckets apparently do not fail immediately when cracks and elongation becomes evident, whether said to have been caused by FOD or Hydrogen Embrittlement. 3,000 hour inspections appear routine, with 1,000 hour inspections being considered.
Maintenance Mode consisted of a dual electrode spark plug fitted to engine inspection ports that allow the inspections of individual rows of compressor blades or turbine buckets.
A central electrode, projecting “X” inches inside the engine housing, is centered inside a circular electrode, mounted flush with the engine housing. An electronic spark is passed between the electrodes, with a gap of “X” inches, while blades or buckets have a tip gap clearance inside the engine of “3X” inches.
When the blades/buckets start cracking, centrifugal force moves the tips outward. It is worth noting that blades/buckets function as in-line springs, with their crystals able to flex ever so slightly. How much they extend at 80% of max RPM is the basis of my warning system. Of course this “3X” blade tip gap inside the engine dimension varies with different engines.
The frequency of the spark could be varied to correspond to near engine RPM, allowing each blade or bucket tip gap to be examined for its extension in apparent slow motion.
When a cracking blade/bucket extends more than “X” distance, it will be closer to the central electrode than the “X” distance to the circular electrode. The spark will then travel to the tip of the elongated blade/bucket, not to the second electrode. The absence of spark to the second electrode would be noted. With the addition of a timing feature, any elongated blade/bucket could be located exactly where it may be found on the hub.
I had envisioned an aircraft taxiing to a maintenance hanger, an analysis consul plugged into a fitting on the engine and all blades and buckets, even bearings, checked for elongation or wear in a relatively short period of time. Then, if a blade or bucket or bearing was found to be elongated or worn, the engine could be promptly exchanged and sent to overhaul, and the aircraft returned to service; a more cost effective system than before.
In-Flight Mode consists of the same dual electrode spark plug with the main electrode still projecting into the engine housing “X” distance; now acting as a rub point. When a blade or bucket tip in any row of any engine extends more than ”2X” distance, it will touch the rub point. This grounds a circuit, sending an individual engine's signal to a warning light in the cockpit. The pilot then takes action as is useful. Only one light needed per engine.
Hundreds of cracked and elongated blades that have rubbed the inside of their engine housings have been found during overhaul, indicating hours of operation after elongation. So the pilot might take note of an engine with a blade or bucket elongating, reduce power on the identified engine, try to avoid compressor stall, and make a safe landing.
The in-flight warning system was in my suggestion to USAF/WPADC in 1959. Whereas the Maintenance Mode Model was what I was trying to develop before I testified in the Senate aviation safety committee hearing. Then I was effectively prevented from doing more. My hope is that someone might be able to convince an engine manufacturer to install what I was told was a similar warning system into engines used in-flight and save lives.
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