NTSB Recommendation on JT15D Failure to Meet Certification Bird Strike Requirements

NTSB Recommendation on JT15D Failure to Meet Certification Bird Strike Requirements

​The US National Transportation Safety Board (NTSB) has issued a safety recommendation urging Transport Canada (TC) to take action to prevent catastrophic failure of Pratt & Whitney Canada (PWC) JT15D-5 engines following a bird strike or foreign object ingestion.  This follows three NTSB investigations of what they call ‘incidents’ involving Beechcraft Beechjet 400A aircraft where failed fan blades breached engine cases and cowlings after bird strikes.  The NTSB note these involved ingestion of “species well below the weight required for certification testing”.

The ‘Incidents

31 July 2009 Beechjet 400A N679SJ immediately lost power from one engine after ingesting at least one bird during take off.  The pilots successfully rejected the take off.

Damage to N679SJ after JT15D Fan Blade Release (Credit: NTSB)

Damage to N679SJ after JT15D Fan Blade Release (Credit: NTSB)

All but one of the fan blades fractured and and that the inlet duct had separated from the front of the engine.

Damage to N679SJ after JT15D Fan Blade Release (Credit: NTSB)

Damage to N679SJ after JT15D Fan Blade Release (Credit: NTSB)

Bird remains from a “1.5-2 lb” yellow-crowned night-heron were found on the runway and in the right engine.  The NTSB say:

Examination of the engine revealed that the engine spinner had separated from the engine due to the bird strike and entered the engine, resulting in the liberation of the fan blades and subsequent failure of the engine. The engine was certified to ingest a four-pound goose. However, the certification requirements in place at the time did not require the spinner to be tested during the certification process. The certification requirements were later updated to require testing of the spinner.

At the time the NTSB determined that “contributing to the incident were the inadequate engine bird strike certification requirements in place at the time the engine was certified”.

13 March 2014 Beechjet 400A, N193BJ, was damaged following a bird strike shortly after take off from Greater Rochester International Airport, NY. The flight crew declared an emergency and safely returned to the airport. Extensive damage to the engine fan and inlet cowl occurred.

Damage to N193BJ after JT15D Fan Blade Release (Credit: NTSB)

Damage to N193BJ after JT15D Fan Blade Release (Credit: NTSB)

The joint between the intake casing and intermediate casing failed.

Damage to Fan / Intermediate Case Joint (Credit: NTSB)

Damage to Fan / Intermediate Case Joint (Credit: NTSB)

The compressor cowl had multiple holes and the right wing had an impact mark forward of the engine fan case.

The bird remains were identified to be a herring gull. The average mass of this species is 1085g (2.39lbs).

This aircraft is currently up for sale (minus the failed engine).

Damage to N193BJ after JT15D Fan Blade Release (Credit: Avclaims)

Damage to N193BJ after JT15D Fan Blade Release (Credit: Avclaims)

13 May 2014 Beechjet 400A N412GJ, suffered an engine fire following a bird strike during the take off roll, also at Sugar Land Regional Airport, Texas. The flight crew rejected the take off and successfully discharged both fire bottles.  The fan blades had all fractured near the root. The nacelle had multiple penetrations.  Afterwards fuel was found leaking from the engine nacelle and from a puncture in the right wing fuel tank.

Damage to N412GJ after JT15D Fan Blade Release (Credit: NTSB)

Damage to N412GJ after JT15D Fan Blade Release (Credit: NTSB)

Recovered bird feathers were identified by the Smithsonian Feather Identification Lab as from a yellow-crowned night-heron. Their average mass is now qouted as “683g (1.51lbs)” by the NTSB.

Certification History

The JT15D-5 was certified under the standard current in October 1980 which stated that at take off power the engine must be capable of ingesting a 4 lb bird without catching fire (as happened in the third case), experiencing an uncontained failure (as happened in all three), generating loads greater than those ultimate loads specified or losing the capability of being shut down”.

The NTSB say that:

PWC provided a copy of Engineering Report No. 1105, “JT15D-5 Certification 4 Pound Ingestion Test.”

The report stated that a four pound bird was propelled at a velocity equivalent to an airplane’s maximum climb speed into a JT15D-5 engine that was running at maximum cruise power.

The report stated that all of the fan blades remained intact, but 5 of the 47 blades were noticeably bent…The report stated that the containment case remained intact.

The report concluded that the test successfully demonstrated the four pound bird ingestion test as required by 14 CFR 33.77(a) by not catching fire, not bursting, not exceeding the engine mount loads, or losing the ability to be shut down.

NTSB Analysis 

The NTSB explain:

Materials analysis on recovered fan blade fragments determined that all separated blades failed due to overload and no evidence of fatigue was identified. A series of modal tests including: tap hammer, high energy shaker, and computer model simulation were conducted at the system and component level. The tests identified a four nodal diameter coincidence at about 72% N1 speed. When sufficient energy is imparted to the engine fan during a foreign object ingestion event like a birdstrike, it may result in a rub induced excitation and subsequent fan blade separation and catastrophic engine failure.

The natural resonance identified during PWC’s testing only occurs in JT15D-5 engines installed on Beechjet 400A airplanes.

Safety Action

The NTSB report that

In response to these findings, PWC is evaluating inlet case damping options to attenuate the identified natural frequency and plans to complete the design change and introduce the modification to the fleet in 2017.

We are nonetheless concerned about the potential for catastrophic engine failure resulting in blade fragments that impact critical structure or lead to engine fire and believe that this risk warrants required corrective action.

The NSTB note that a Service Bulletin alone does note make action mandatory.

NTSB Safety Recommendation

To Transport Canada: Require that all JT15D-5 engines installed on Raytheon/Hawker Beechjet 400A airplanes be modified to dampen the rub-induced excitation that can occur between the engine fan and fan case during birdstrike or foreign object ingestion events. (A-17-7)

JT15D Background

JT15D-5 (Credit: PWC)

JT15D-5 (Credit: PWC)

Thirteen JT15D models have been produced in three series (JT15D-1, JT15D-4 and JT15D-5) ranging from 2,200 to 3,400 lb thrust.  The certification application date for the JT15D-1 was 2 June 1969 and for the -5 was 5 January 1982).  The two spool engine has a single stage fan and booster stage driven by a single low pressure turbine stage, a single stage high pressure centrifugal compressor driven by a single high pressure turbine stage and a reverse flow compressor.

Other Safety Resources

Aerossurance is pleased to sponsor the 2017 European Society of Air Safety Investigators (ESASI) 8th Regional Seminar in Ljubljana, Slovenia on 19 and 20 April 2017.  Registration is just €100 per delegate. To register for the seminar please follow this link.  ESASI is the European chapter of the International Society of Air Safety Investigators (ISASI).

Aerossurance is also pleased to sponsor an RAeS HFG:E conference at Cranfield University on 9 May 2017, on the topic of Staying Alert: Managing Fatigue in Maintenance.  This event will feature presentations and interactive workshop sessions.

Aerossurance has extensive air safety, engine design & certification, propulsion system integration, airworthiness, operations, human factors, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com