NTSB: Stronger 737NG Engine Cowls Needed


Reinforced fan cowls for Boeing 737 Next Generation airliners are among the NTSB’s recommendations after analyzing the April 2018 incident with a Southwest Airlines flight that resulted in one passenger fatality. The left engine on this 737-700 failed 30 minutes after the flight left New York’s LaGuardia Airport, with fragments from the first-stage fan tearing parts of the cowling off the wing and impacting the fuselage, where it caused a window to depart the airframe. The passenger at that window seat succumbed to her injuries. 

In a preamble to seven specific recommendations, the NTSB says that the “left engine failure occurred when one of the fan blades fractured at its root (referred to as a fan-blade-out [FBO] event). The fan blade fractured due to a low-cycle fatigue crack that initiated in the dovetail (part of the blade root), which remained within a slot of the fan disk.” It notes that when the engine was last overhauled in 2012, it was done according to acceptable standards and practices at the time and it was unlikely the crack would have been found through testing. After another blade failure in 2016, engine manufacturer CFM International came up with an on-wing inspection intended to find cracks, and this test is now conducted with periodic blade “relubrication” on the CFM56.

“This accident demonstrates that a fan blade can fail and release differently than that observed during engine certification testing and accounted for in airframe structural analyses,” said NTSB Chairman Robert Sumwalt in a press release accompanying the report. “It is important to go beyond routine examination of fan blades; the structural integrity of the engine nacelle components for various airframe and engine combinations needs to be ensured.”

The NTSB’s narrative paints a vivid picture of the aftermath. “Portions of the left engine inlet and fan cowl separated from the airplane, and fragments from the inlet and fan cowl struck the left wing, the left-side fuselage, and the left horizontal stabilizer. One fan cowl fragment impacted the left-side fuselage near a cabin window, and the window departed the airplane, which resulted in a rapid depressurization.” Kudos to the flight crew, which declared an emergency and landed in Philadelphia only 17 minutes after the engine failure. 

At the heart of the matter, the NTSB found that the aftereffects of the blade failure were different and more pronounced than expected from certification testing. “During the accident sequence, the fan blade fragments traveling forward of the fan case had a trajectory angle that was greater than that observed during the CFM56-7B engine FBO containment certification tests,” says the report. “Also, the inlet damage caused by the forward-traveling fan blade fragments was greater than that observed during the engine FBO containment certification tests and accounted for in Boeing’s 737-700 certification analyses (which used the state-of-the-art analytical modeling tools that were available at the time).” The NTSB noted that the incident airplane had the blade fragments penetrate the cowling at the 6 o’clock position, while the certification tests were carried out with the blade impacting at the 12 o’clock position. 

Of the seven recommendations set out by the NTSB, three of them involve Boeing determining the most critical failure location on the fan cowl and reinforcing it to withstand the impact, then fitting them to new-production NGs, and then ensuring that the fleet is retrofitted to the new standard. The NTSB also suggests that the FAA should expand “Part 25 and 33 certification requirements to mandate that airplane and engine manufacturers work collaboratively to (1) analyze all critical fan blade impact locations for all engine operating conditions, the resulting fan blade fragmentation, and the effects of the fan-blade-out-generated loads on the nacelle structure and (2) develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.”

The NTSB also criticized the cabin crew for not taking their jumpseats during the emergency landing. According to the NTSB, “all three flight attendants were seated on the cabin floor, which was contrary to the procedures in the SWA flight attendant manual that required flight attendants to occupy their assigned jumpseats during a planned emergency landing. One of the flight attendants who was stationed in the forward cabin reported that she did not have time to return to her jumpseat; the other flight attendant assigned to the forward cabin sat on the floor in the aft galley. Thus, the forward dual-position flight attendant jumpseat was unoccupied during the landing. The flight attendant assigned to the aft cabin also sat on the floor in the aft galley because her jumpseat was occupied by a passenger from row 14 (who had relocated to the aft cabin so that the injured passenger could receive medical care) and an SWA company employee.”

The NTSB notes that this was a full flight and so to make room for individuals trying to resuscitate the stricken woman—who had been pulled partially out of the fractured window—the passengers in the adjoining seats had to go somewhere. Which leads to this recommendation from the NTSB: “Develop and issue guidance on ways that air carriers can mitigate hazards to passengers affected by an in-flight loss of seating capacity.” Noting that there was no specific guidance outlined in any of the FAA’s regulations or advisory circulars, the NTSB nevertheless said that “such guidance would help air carriers implement procedures to mitigate hazards to passengers resulting from an in-flight loss of seating capacity.”

KITPLANES Editor in Chief Marc Cook has been in aviation journalism for more than 30 years. He is a 4000-hour instrument-rated, multi-engine pilot with experience in nearly 150 types. He’s completed two kit aircraft, an Aero Designs Pulsar XP and a Glasair Sportsman 2+2, and currently flies a 2002 GlaStar.

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  1. Being critical of the flight attendants of their actions during an actual emergency is a bit out of line for the NTSB. From what I have heard they did what they had to to meet the emergency and to, as best they could, take care of the critically injured passenger before landing. There is a reason the FAR’s allow crews to take whatever action needed to meet that emergency even if it violates FAR’s.

    As far as the recommendation for airlines to take into account the loss of seats in an emergency, I see that recommendation going nowhere fast.

    I find it interesting that the NTSB found that the failure paths the fan blades took were not as predicted per the certification tests. With the issues raised with the MAX investigation on pilot reaction to sudden unexpected problems seems like things happening not as predicted in aviation are more common than most people realize.

    • Somewhere in a past Avweb blog, there was talk of having passengers STAND to cram ’em in tighter. Boeing could install poles in the cabin floor that’d automatically deploy with the flick of a switch by a crew member. Then, all the displaced passengers could run to their nearest empty passenger pole which would have a strap they could hold onto. Because all the PAX might run aft or forward, a system similar to MCAS would have to be installed to compensate for the CG change automatically.

      Maybe Boeing could go over to the M1A1 tank factory and get some of that armor they use?

      Are we actually paying the NTSB employees to produce this sort of crap ? Pontification should be uncompensated !

    • “…the NTSB found that the failure paths the fan blades took were not as PREDICTED (my emphasis) per the certification tests.”

      That’s not what the story said. THIS is what the story said (my emphasis):
      “During the accident sequence, the fan blade fragments traveling forward of the fan case had a trajectory angle that was greater than that OBSERVED during the CFM56-7B engine FBO containment certification tests.”

      It went on to say:
      “The NTSB noted that the incident airplane had the blade fragments penetrate the cowling at the 6 o’clock position, while the certification tests were carried out with the blade impacting at the 12 o’clock position.”

      Then the NTSB recommended all-aspect testing of FBO events. That’s a major departure from current certification requirements.

  2. Containment efforts are necessary, but will NEVER be 100% effective because of the energy involved; especially when the blade separates at the root. A practical solution (that nobody wants to hear) is to remove windows at any location susceptible to such damage and replace with sheet metal. You STILL need containment efforts.

    • “Containment efforts are necessary…”
      While they may be practical, they’re not “necessary.” The OBJECTIVE of containment of FBO events is the prevention of damage to the aircraft structure. Containment is just one means of acheiving that end.

      Armoring proximate structure has drawbacks – chiefly weight penalties – but it addresses the actual objective, directly.

  3. Boeing should try to increase the containment strength by using correctly done Deep Cryogenic Treatment (DCT). Design changes would be minimal, and it would be easy to implement perhaps even on existing structures. Also, if fibre materials are used for “armoring” the containment vessel, it has been proven that DCT can greatly increase their strength and penetration resistance.

    Regarding the blades, DCT has shown to greatly improve fatigue resistance. Again, probably no design changes other than the use of DCT.

    DCT is basically a heat treating process that uses cold to modify the crystal structure. After all it is all heat when you are above absolute zero. Unfortunately both Boeing and most engine makers do not understand the physics behind the process.