+----------------+---+-----------------------------------------------------+
| **Accident**   |   | National Airlines DC-10, Flight 27                  |
+----------------+---+-----------------------------------------------------+
|                |   |                                                     |
+----------------+---+-----------------------------------------------------+
| **Location**   |   | Albuquerque, New Mexico                             |
+----------------+---+-----------------------------------------------------+
|                |   |                                                     |
+----------------+---+-----------------------------------------------------+
| **Date**       |   | November 3, 1973                                    |
+----------------+---+-----------------------------------------------------+
|                |   |                                                     |
+----------------+---+-----------------------------------------------------+
| **Summary**    |   | On November 3, 1973, while National Airlines DC-10, |
|                |   | flight 27, from Miami to San Francisco, was         |
|                |   | cruising at 39,000 feet, the No. 3 engine fan       |
|                |   | assembly completely disintegrated. See [Figure      |
|                |   | 1](#Fig_1)[]{#Return_from_Fig_1 .anchor}.           |
|                |   |                                                     |
|                |   | The No. 3 inlet and fan case separated              |
|                |   | simultaneously with fan blade liberation, allowing  |
|                |   | the blades to impact the airplane at high speed.    |
|                |   |                                                     |
|                |   | Flight 27 was a McDonnell Douglas DC-10-10 with     |
|                |   | General Electric Aircraft Engines (GEAE) CF6-6D     |
|                |   | turbofan engines installed ([Figure                 |
|                |   | 2](#Fig_2)[]{#Return_from_Fig_2 .anchor}). See      |
|                |   | [Figure 3](#Fig_3)[]{#Return_from_Fig_3 .anchor}    |
|                |   | for a close up of the CF6-6 wing installation. See  |
|                |   | [Figure 4](#Fig_4)[]{#Return_from_Fig_4 .anchor}    |
|                |   | for a cutaway view of the CF6-6 engine.             |
|                |   |                                                     |
|                |   | The liberated blades struck the fuselage, right     |
|                |   | wing, engines No. 1 and 2, and caused a cabin       |
|                |   | window to fail. See [Figure                         |
|                |   | 5](#Fig_5)[]{#Return_from_Fig_5 .anchor} and        |
|                |   | [Figure 6](#Fig_6)[]{#Return_from_Fig_6 .anchor}.   |
|                |   |                                                     |
|                |   | This resulted in the expulsion of one passenger     |
|                |   | through the opening during rapid decompression.     |
|                |   | Many passenger oxygen masks failed to deploy,       |
|                |   | causing a panic among the remaining passengers.     |
|                |   |                                                     |
|                |   | The engine debris caused damage to two of the       |
|                |   | aircraft\'s three hydraulic systems resulting in    |
|                |   | loss of all hydraulic fluid for both systems.       |
|                |   |                                                     |
|                |   | At the time of the No. 3 fan failure [(Figure       |
|                |   | 7)](#Fig_7)[]{#Return_from_Fig_7 .anchor}, the      |
|                |   | pilot and flight engineer were \"experimenting\"    |
|                |   | with the relationship between the engine\'s fan     |
|                |   | speed (N1) signals and the automatic flight control |
|                |   | systems. The pilot and flight engineer appeared to  |
|                |   | be curious as to where the autopilot/auto throttle  |
|                |   | received its engine speed signal (i.e., N1 gauge or |
|                |   | N1 tachometer generator). The crew reported that    |
|                |   | their \"experiment\" was conducted as follows:      |
|                |   |                                                     |
|                |   | 1.  Autopilot selected in airspeed mode - 257 KIAS. |
|                |   |                                                     |
|                |   | 2.  Flight engineer pulled circuit breakers for N1  |
|                |   |     tachometers on engine numbers 1, 2, and 3.      |
+----------------+---+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| **Summary      | 3.  Target airspeed reduced by 5 knots.             |
| (continued)**  |                                                     |
|                | 4.  When throttles retarded, pilot disengaged auto  |
|                |     throttle.                                       |
|                |                                                     |
|                | 5.  Crew then heard explosion and cabin             |
|                |     decompression.                                  |
|                |                                                     |
|                | The crew declared an emergency and executed an      |
|                | emergency descent. There was an uneventful landing  |
|                | at Albuquerque, New Mexico. The oil tank in the No. |
|                | 1 engine had been penetrated by fan blade debris    |
|                | and upon landing had lost all oil, and the No. 1    |
|                | engine constant speed drive generator line was      |
|                | severed.                                            |
|                |                                                     |
|                | Engine No. 2 ingested fan debris and sustained      |
|                | minor foreign object damage.                        |
|                |                                                     |
|                | The failure of a substantial number of oxygen masks |
|                | to deploy appeared to result from multiple          |
|                | electrical failures.                                |
|                |                                                     |
|                | Analysis of cockpit voice recorder data shows that  |
|                | the No. 3 engine accelerated from 97% N1 to 100%    |
|                | N1, fluctuated for a few seconds and failed at      |
|                | 100%; the flight data recorder was inoperative.     |
|                | Engines No. 1 and 2 also accelerated from 97% N1 to |
|                | a maximum of 107% N1.                               |
|                |                                                     |
|                | See [Figure 8](#Fig_8)[]{#Return_from_Fig_8         |
|                | .anchor} of the No. 1 engine cowl showing impact    |
|                | damage from the No. 3 engine debris. Engine debris  |
|                | penetrated the oil tank resulting in total loss of  |
|                | engine oil upon landing.                            |
|                |                                                     |
|                | The investigation centered on vibratory             |
|                | characteristics of the fan blades and fan case.     |
|                | There was a debate on the role of the inlet on the  |
|                | blade/case relationship that asked: \"Did the inlet |
|                | failure cause tip rub?\" or \"Did tip rub cause the |
|                | inlet failure?\"                                    |
|                |                                                     |
|                | The initial triggering event was never completely   |
|                | agreed upon by all parties. However, the failure    |
|                | mechanism was sufficiently understood so that       |
|                | corrective actions to prevent additional            |
|                | occurrences could be defined.                       |
|                |                                                     |
|                | The identified failure mechanism involved a fan     |
|                | case \"6-wave\" mode traveling at 1/2 fan speed in  |
|                | the direction of the fan rotation. See [Figure      |
|                | 9](#Fig_9)[]{#Return_from_Fig_9 .anchor}.           |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| **Summary      | The fan blade assembly also exhibited a             |
| (continued)**  | fore-and-aft rocking vibratory mode [(Figure        |
|                | 10)](#Fig_10)[]{#Return_from_Fig_10 .anchor} which  |
|                | has a traveling wave shape at 1/2 fan speed in an   |
|                | opposite direction to the fan.                      |
|                |                                                     |
|                | A vibratory mode shape is the way in which the      |
|                | object deflects at its natural frequency.           |
|                |                                                     |
|                | An example of natural frequency and mode shape is   |
|                | given in the case of a guitar string. When struck,  |
|                | the string vibrates at a certain frequency and      |
|                | attains a deflected shape. The frequency can be     |
|                | noted by the pitch coming from the string.          |
|                | Different string geometrics (length and diameter)   |
|                | lead to different natural frequencies or notes.     |
|                |                                                     |
|                | It was generally agreed that two conditions had to  |
|                | be present for a massive engine failure such as     |
|                | this to result:                                     |
|                |                                                     |
|                | 1.  A coincident and coupled fan case wave and      |
|                |     blade \"rocking\" wave mode, and                |
|                |                                                     |
|                | 2.  Fan blade tip rub.                              |
|                |                                                     |
|                | The Campbell diagram in [Figure                     |
|                | 11](#Fig_11)[]{#Return_from_Fig_11 .anchor}         |
|                | illustrates the fan case and blade vibratory modes. |
|                |                                                     |
|                | It is important to note that although the flight    |
|                | crew was experimenting with the aircraft systems    |
|                | and not following an approved procedure, the root   |
|                | cause of the failure was not the improper flight    |
|                | crew actions in the cockpit, but a design problem   |
|                | with the engine.                                    |
|                |                                                     |
|                | The maximum speed of the fan rotor on the No. 3     |
|                | engine when it failed was determined to be          |
|                | approximately 100% which is below the maximum red   |
|                | line speed of 111%. Had the No. 3 engine not failed |
|                | on the National Airlines flight, there would have   |
|                | eventually been a similar failure on that model     |
|                | engine at some future time in revenue service.      |
|                |                                                     |
|                | It is also important to recognize and be on guard   |
|                | during the investigative phase of                   |
|                | accidents/incidents for red herrings or preliminary |
|                | findings/observations that may not be relevant and  |
|                | may detract from determining the true root cause    |
|                | and necessary corrective actions for the event.     |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| **Accident     | The National Transportation Safety Board (NTSB)     |
| Board          | determined that the probable cause of this accident |
| Findings**     | was the disintegration of the No. 3 engine fan      |
|                | assembly as a result of interaction between the fan |
|                | blade tips and the fan case. See [Figure            |
|                | 12](#Fig_12)[]{#Return_from_Fig_12 .anchor}. The    |
|                | fan-tip rub condition was caused by the             |
|                | acceleration of the engine to an abnormally high    |
|                | fan speed, which initiated a multiwave vibration    |
|                | resonance within the fan section of the engine.     |
|                |                                                     |
|                | The precise reason or reasons for the acceleration  |
|                | and the onset of the destructive vibration could    |
|                | not be conclusively determined.                     |
|                |                                                     |
|                | The NTSB identified 14 findings for this accident   |
|                | that can be found in the National Airlines          |
|                | Albuquerque NTSB Accident Report, NTSB-AAR-75-2.    |
|                | View the report at:                                 |
|                | <http://case.pr.erau.edu/reports/U                  |
|                | S_reports/1970/1973.11.03_NationalAirlines_27.pdf>. |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Accident     | The NTSB issued nine safety recommendations to the  |
| Board          | FAA. View the detailed NTSB safety recommendations  |
| Rec            | and corresponding FAA responses at:                 |
| ommendations** | <http://fa                                          |
|                | alessons.workforceconnect.org/rawmedia_repository/5 |
|                | e75d016_3085_4755_9383_e2f186d1e25f?/document.pdf>. |
|                |                                                     |
|                | Three of the recommendations (A-73-116, -117, and   |
|                | -118) pertain to the inspection and maintenance of  |
|                | digital flight data recorders. The digital flight   |
|                | data recorder from the National Airlines airplane   |
|                | was found to be defective and no meaningful data    |
|                | could be retrieved.                                 |
|                |                                                     |
|                | Five recommendations concern the passenger and      |
|                | portable oxygen systems installed in the DC-10. The |
|                | accident revealed deficiencies relative to the      |
|                | installation of the cabin oxygen generating units,  |
|                | functionality of the system and accessibility of    |
|                | portable oxygen units in the cabin.                 |
|                |                                                     |
|                | One final recommendation pertains to the assessment |
|                | of aircraft damage by flight crews during in-flight |
|                | emergencies. The NTSB investigation identified that |
|                | National Airlines did not have a procedure for crew |
|                | members to assess the structural damage to the      |
|                | airplane in an emergency situation. After the       |
|                | emergency was under initial control, the cockpit    |
|                | crew did not assess the extent of damage to the     |
|                | airplane or the condition in the cabin.             |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| **Accident     | Due to the prompt and effective actions taken by    |
| Board          | the FAA, GEAE, McDonnell Douglas, and airlines      |
| R              | flying the DC-10, no recommendations regarding      |
| ecommendations | engine installation were necessary.                 |
| (continued)**  |                                                     |
|                | Immediately following the accident, the FAA issued  |
|                | a telegraphic Airworthiness Directive (AD) that     |
|                | applied to all DC-10 aircraft. The AD required      |
|                | inspection of the engine nose cowl mounting         |
|                | integrity and correction of any possible            |
|                | deficiencies. Also, it was recognized early that    |
|                | fan-tip rub was a necessary condition in the        |
|                | sequence of events that brought about the loss of   |
|                | the fan blades.                                     |
|                |                                                     |
|                | As a preventive measure against the recurrence of   |
|                | this type of condition, the fan blade tip-to-shroud |
|                | clearances were increased. As backup for the        |
|                | possibility of blade-tip rub even after the tip     |
|                | clearance was modified, an extensive development,   |
|                | testing, and production program was established to  |
|                | increase the capabilities of the blade retention    |
|                | devices.                                            |
|                |                                                     |
|                | The blade retention devices were redesigned with    |
|                | increased capability of more than 3 times the       |
|                | original configuration. These modified blade        |
|                | retention devices were incorporated in all revenue  |
|                | service engines.                                    |
|                |                                                     |
|                | In the accident report, the NTSB addressed the      |
|                | flight crew performance of an experiment with the   |
|                | autothrottle/speed control system. The NTSB         |
|                | stressed that the operator and the pilot in command |
|                | should be fully cognizant of their operational      |
|                | responsibilities to conduct the flight in a         |
|                | professional manner and not to conduct experiments  |
|                | with aircraft systems in which they have not        |
|                | received specific training or instruction.          |
|                |                                                     |
|                | The individual NTSB safety recommendations can be   |
|                | found in the National Airlines Albuquerque NTSB     |
|                | Accident Report, NTSB-AAR-75-2, at:                 |
|                | <http://case.pr.erau.edu/reports/U                  |
|                | S_reports/1970/1973.11.03_NationalAirlines_27.pdf>. |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Unsafe       | The unsafe conditions that contributed to this      |
| Conditions**   | incident included:                                  |
|                |                                                     |
|                | 1.  The destructive \"interactive\" fan blade and   |
|                |     fan case vibration mode within the engine       |
|                |     operating envelope.                             |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Unsafe       | 2.  Engine design and certification standards that  |
| Conditions     |     require the engine structure to contain damage  |
| (continued)**  |     resulting from a single rotor blade failure at  |
|                |     the outermost retention groove. This engine     |
|                |     failure resulted in a number of complete fan    |
|                |     blades being released from the fan disk at high |
|                |     speed coincident with the fan containment case  |
|                |     separating from the engine. This reduced its    |
|                |     effectiveness in containing the engine debris   |
|                |     and minimizing the hazard to the aircraft. High |
|                |     energy rotors and uncontained engine debris can |
|                |     cause a hazard to the aircraft. Aircraft safety |
|                |     for this type of threat is provided by a        |
|                |     combined effort of precluding the uncontained   |
|                |     failures and by mitigating the effects to       |
|                |     aircraft in the event of a failure. In this     |
|                |     failure scenario, the aircraft safety depended  |
|                |     upon the DC-10-10 design precautions that were  |
|                |     taken to minimize the hazards from uncontained  |
|                |     engine debris.                                  |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Design and   | The following design and safety assumptions are     |
| Safety         | relevant to this accident:                          |
| Assumptions**  |                                                     |
|                | 1.  There were no harmful interacting vibratory     |
|                |     modes existing within the engine operating      |
|                |     range.                                          |
|                |                                                     |
|                | 2.  Engine design and certification standards       |
|                |     require the engine structure to contain damage  |
|                |     resulting from a single rotor blade failure at  |
|                |     the outermost retention groove. The National    |
|                |     Airlines engine failure resulted in a number of |
|                |     complete fan blades being released from the fan |
|                |     disk at high speed and the engine structure     |
|                |     (fan containment case) separating from the      |
|                |     engine which reduced its effectiveness in       |
|                |     containing the fan blades and minimizing the    |
|                |     hazard to the aircraft. This failure mode       |
|                |     exceeded design and certification standards for |
|                |     the engine and aircraft safety relied upon      |
|                |     aircraft design considerations for minimizing   |
|                |     hazards caused by uncontained engine debris.    |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Precursors** | There are two precursors that have been identified  |
|                | for the National Airlines Flight 27 accident.       |
|                |                                                     |
|                | First, American Airlines experienced a similar      |
|                | engine failure during test cell operation of a CF6  |
|                | engine on November 15, 1972; the bellmouth [(Figure |
|                | 13)](#Fig_14)[]{#Return_from_Fig_13 .anchor} and    |
|                | exhaust cone separated and all the fan blades were  |
|                | liberated.                                          |
|                |                                                     |
|                | This was a precursor because the same failure mode  |
|                | as the National Airlines event was experienced but  |
|                | was not fully understood and the risk for a repeat  |
|                | event to a revenue service engine was not           |
|                | recognized.                                         |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Precursors   | The failure investigation put considerable focus on |
| (continued)**  | the cause of the failure due to possible improper   |
|                | installation of the bellmouth to engine attachments |
|                | bolts by American Airlines. This was eventually     |
|                | shown to be irrelevant to the failure. This is      |
|                | another example of a \"red herring\" that can       |
|                | detract from determining the true root cause.       |
|                |                                                     |
|                | GEAE experienced a similar engine failure during    |
|                | test cell operation of a highly instrumented CF6    |
|                | engine (engineering test) on January 12, 1973.      |
|                |                                                     |
|                | This failure was initiated by the rubbing between   |
|                | the fan rotor and casing and resulted in damage     |
|                | similar to the other occurrences.                   |
|                |                                                     |
|                | This was a precursor because the same failure mode  |
|                | as the National Airlines event and the American     |
|                | Airlines event was experienced and the risk for a   |
|                | repeat event to a revenue service engine was not    |
|                | recognized. This engine was instrumented and was    |
|                | undergoing engineering tests in part as a result of |
|                | the previous American Airlines test cell failure.   |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| **Resulting    | There were no specific ACs resulting from this      |
| ACs,           | accident. However, as a result of the events of     |
| Regulatory     | November 3, 1973, the following changes were made:  |
| Guidance and   |                                                     |
| Policy**       | **Engine/Airplane Design Changes:**                 |
|                |                                                     |
|                | Number of inlet cowl brackets increased from 12 to  |
|                | 21.                                                 |
|                |                                                     |
|                | Blade axial retention increased from 18,000 to      |
|                | 60,000 pounds-force ultimate.                       |
|                |                                                     |
|                | Fan blade/case tip clearance increased.             |
|                |                                                     |
|                | **GEAE Design Practice:**                           |
|                |                                                     |
|                | GEAE engine design practice was updated to          |
|                | incorporate lessons learned regarding fan rotor and |
|                | case harmful interacting vibratory modes.           |
|                |                                                     |
|                | **Advisory Material:**                              |
|                |                                                     |
|                | This accident provided data and information         |
|                | relative to engine fragment spread angles that were |
|                | used for development of FAA Order 8110.11, titled   |
|                | \"Design Considerations for Minimizing Damage       |
|                | Caused by Uncontained Aircraft Turbine Engine Rotor |
|                | Failures,\" dated November 19, 1975.                |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| **Resulting    | The information in this Order was subsequently      |
| ACs,           | revised and issued in AC 20-128, which you can view |
| Regulatory     | at:                                                 |
| Guidance and   | <http://www.airweb.faa.gov/Regulatory_and_Guid      |
| Policy         | ance_Library/rgAdvisoryCircular.nsf/0/536BFC2BFEABC |
| (continued)**  | 079862569AF006AAE73?OpenDocument&Highlight=20-128>. |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| *              | The following airworthiness directives were issued  |
| *Airworthiness | as a result of this event. View the ADs in the      |
| Directives     | FAA\'s Regulatory and Guidance Library at:          |
| Issued**       | <                                                   |
|                | http://www.airweb.faa.gov/Regulatory_and_Guidance_L |
|                | ibrary/rgWebcomponents.nsf/HomeFrame?OpenFrameSet>. |
|                |                                                     |
|                | **Engine:**                                         |
|                |                                                     |
|                | No airworthiness directives were issued on the      |
|                | engine.                                             |
|                |                                                     |
|                | **Airplane:**                                       |
|                |                                                     |
|                | 1.  Telegraphic AD issued November 7, 1973, that    |
|                |     applies to DC-10-10, -30, -30F series aircraft. |
|                |     The AD required inspection of nose cowl bolts   |
|                |     on DC-10 wing-mounted engines. AD 73-26-04      |
|                |     became effective on December 19, 1973, and      |
|                |     incorporates the requirements of the            |
|                |     telegraphic AD.                                 |
|                |                                                     |
|                | 2.  AD 75-04-03 became effective on February 14,    |
|                |     1975, and requires improvements to the DC-10    |
|                |     cabin oxygen mask deployment system. Before the |
|                |     National Airlines accident, the FAA was working |
|                |     with McDonnell Douglas on improving the DC-10   |
|                |     cabin oxygen mask deployment system. This       |
|                |     accident resulted in the higher prioritization  |
|                |     of this activity and contributed to the         |
|                |     issuance of AD 75-04-03.                        |
+----------------+-----------------------------------------------------+
|                |                                                     |
+----------------+-----------------------------------------------------+
| ###### L       | The following lessons should be learned from this   |
| essons Learned | event:                                              |
|                |                                                     |
|                | 1.  Fan blades and fan case can have harmful        |
|                |     vibratory modes.                                |
|                |                                                     |
|                | 2.  Previous test cell failures were precursor      |
|                |     events for the accident.                        |
|                |                                                     |
|                | 3.  Precursor events must be resolved or risk being |
|                |     repeated.                                       |
+----------------+-----------------------------------------------------+
| ######         |                                                     |
+----------------+-----------------------------------------------------+

+----------------+-----------------------------------------------------+
| ######         | 4.  The engine failure exceeded the design and      |
|  Lessons Learn |     certification standards for engine rotor blade  |
| ed (continued) |     containment. However, the aircraft design       |
|                |     precautions were successful in that the damage  |
|                |     sustained by the aircraft was not excessive to  |
|                |     the point where it prevented a successful       |
|                |     emergency landing.                              |
|                |                                                     |
|                | 5.  In the investigative phase of accidents or      |
|                |     incidents, it is important to recognize and be  |
|                |     on guard in the for \"red herrings\" or         |
|                |     preliminary findings/observations that may not  |
|                |     be relevant and may detract from determining    |
|                |     the true root cause and necessary corrective    |
|                |     actions of the event.                           |
|                |                                                     |
|                | 6.  Following the accident, the FAA conducted a     |
|                |     Critical Design Review (CDR) to address         |
|                |     potential DC-10 safety issues related to CF6    |
|                |     engine fan blade fragment ejection. The CDR put |
|                |     considerable focus on fan blade fragment        |
|                |     ejection and its potential hazards to the       |
|                |     aircraft for a specific fan blade problem that  |
|                |     was resulting in frequent fan blade failures in |
|                |     revenue service. This review may have been too  |
|                |     focused on a specific revenue service problem   |
|                |     and did not address system level threats such   |
|                |     as loss of the aircraft hydraulic system due to |
|                |     uncontained engine debris. The investigation    |
|                |     needs to be broad enough to look for other      |
|                |     possible accident precursors.                   |
+----------------+-----------------------------------------------------+

+---------------+------------------------------------------------------+
| **Category**  | > Environmental Mechanical Aging Aircraft            |
|               | >                                                    |
|               | > Avionics Propulsion Human Factors                  |
|               | >                                                    |
|               | > Electrical Auxiliary, other Software               |
|               | >                                                    |
|               | > Interior Airframe Environment                      |
|               | >                                                    |
|               | > Cabin Safety Flight Crew Maintenance, Operations   |
|               | >                                                    |
|               | > Propulsion Installation                            |
|               | >                                                    |
|               | > Regulations, policies, standards                   |
+---------------+------------------------------------------------------+
|               |                                                      |
+---------------+------------------------------------------------------+
| **Application | Design & Certification Regulation, Policy, etc.      |
| of Lessons    |                                                      |
| Learned**     | ADs, TCs, etc. Other:                                |
|               | \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_                 |
|               |                                                      |
|               | Maintenance                                          |
|               |                                                      |
|               | Continued Operational Safety                         |
+---------------+------------------------------------------------------+
|               |                                                      |
+---------------+------------------------------------------------------+
| ###           | National Airlines, Albuquerque New Mexico,           |
| ### Key Words | interactive vibratory modes, rotor-case interaction, |
|               | fan blade, rapid decompression, tip rub.             |
+---------------+------------------------------------------------------+
|               |                                                      |
+---------------+------------------------------------------------------+
| ###### Potent | NTSB PTRS                                            |
| ial Databases |                                                      |
|               | ASRS SDR                                             |
|               |                                                      |
|               | ATOS Other: [ANE-140 and ANM-140L project            |
|               | files]{.underline}                                   |
|               |                                                      |
|               | CSET [AAI -- Office of Accident                      |
|               | Investigation]{.underline}                           |
+---------------+------------------------------------------------------+

[]{#Fig_1 .anchor}**Figure 1: National Flight 27 After the Incident**

[**(Select here to return)**](#Return_from_Fig_1)

![](media/image1.jpeg){width="5.754166666666666in"
height="4.173611111111111in"}

[]{#Fig_2 .anchor}**Figure 2: DC-10 Airplane View Illustrated with
Engine Arrangement**

[(Select here to return)](#Return_from_Fig_2)

![](media/image2.jpeg){width="5.989583333333333in"
height="7.333333333333333in"}

[]{#Fig_3 .anchor}**Figure 3: CF6-6 Wing Installation**

[(Select here to return)](#Return_from_Fig_3)

![](media/image3.jpeg){width="6.491666666666666in"
height="5.014583333333333in"}

[]{#Fig_4 .anchor}**Figure 4: CF6-6 Engine Cut-Away View**

[(Select here to return)](#Return_from_Fig_4)

![](media/image4.jpeg){width="6.495833333333334in" height="4.80625in"}

[]{#Fig_5 .anchor}**Figure 5: National Airlines DC-10 Flight #27, No. 3
Engine; November 3, 1973**

[(Select here to return)](#Return_from_Fig_5)

![](media/image5.jpeg){width="6.0in" height="7.5in"}

[]{#Fig_6 .anchor}**Figure 6: National Airlines DC-10 Flight #27,**

**View of Engine No. 3 Through Missing Cabin Window, Seat #17H, November
3, 1973**

[(Select here to return)](#Return_from_Fig_6)

![](media/image6.jpeg){width="5.034027777777778in"
height="7.250694444444444in"}

[]{#Fig_7 .anchor}**Figure 7: National Airlines DC-10 Flight #27, No. 3
Engine; November 3, 1973**

[**(Select here to return)**](#Return_from_Fig_7)

![](media/image7.jpeg){width="4.625694444444444in"
height="6.786111111111111in"}

[]{#Fig_8 .anchor}**Figure 8: National Flight 27 with a Number 3 Fan
Blade**

**through the Oil Tank of the Number 1 Engine**

[(Select here to return)](#Return_from_Fig_8)

![](media/image8.jpeg){width="6.4944444444444445in"
height="4.540972222222222in"}

[]{#Fig_9 .anchor}**Figure 9: Fan Case "6-Wave" Mode (Front View of
Engine Looking Aft)**

[(Select here to return)](#Return_from_Fig_9)

![](media/image9.jpeg){width="4.489583333333333in" height="3.71875in"}

[]{#Fig_10 .anchor}**Figure 10: Fan Blade Assembly Fore-and-Aft Rocking
Mode**

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![](media/image10.jpeg){width="3.2291666666666665in" height="3.1875in"}

[]{#Fig_11 .anchor}**Figure 11: Example of Campbell Diagram of Fan Case
and Blade Vibratory Modes**

[(Select here to return)](#Return_from_Fig_11)

This figure shows a Campbell diagram of the fan case and blade vibratory
modes. It is for illustrative purposes only and is not to scale. A
Campbell diagram is a method of illustrating the natural frequency of an
object and its common exciting forces.

The designer uses this information to determine the conditions where a
part may be susceptible to high vibrations which can lead to part
failure, and to assure that resonance does not occur during steady state
operation, particularly at high power.

![](media/image11.jpeg){width="4.875in" height="4.145833333333333in"}

[]{#Fig_12 .anchor}**Figure 12: National Airlines DC-10 Flight #27,**

**No. 3 Engine Fan Blade -- Typical; November 3, 1973**

[**(Select here to return)**](#Return_from_Fig_12)

![](media/image12.jpeg){width="5.4840277777777775in"
height="7.115277777777778in"}

[]{#Fig_14 .anchor}**Figure 13: Example of Bellmouth Attached to Front
of Engine for Testing**

[(Select here to return)](#Return_from_Fig_13)

A bellmouth inlet is usually installed on an engine being calibrated in
a ground test stand. This type of inlet is easily attached and removed
from the engine. It is designed with the objective of obtaining very
high aerodynamic efficiency.

The inlet is a bell shaped funnel having carefully rounded shoulders
which offer very little air resistance.

Very accurate engine performance data can be obtained while using the
bellmouth inlet.

![](media/image13.jpeg){width="4.020833333333333in"
height="5.997916666666667in"}