Problems and Solutions
Can you identify the effect of the door latch-pin failure in this case study? The loss of the aft cargo door on the American Airlines DC-10 aircraft will be the focus of this Problems and Solutions article. This real-life case study is being used to practice identifying FMEA elements.
If you haven’t yet read the article “Understanding FMEA Effects – Part 1“, this would be a good time, as it presents fundamental information about the effects of failure in an FMEA.
In an FMEA, which of the following is true about an “effect”? (Select all that apply)
1. An “effect” is the specific reason for the failure.
2. An “effect” is the potential consequence or impact of the failure to the system or end user.
3. An “effect” is the manner in which an item does not accomplish its intended functions.
4. None of the above.
In an FMEA, which of the following is true about an “effect”? (Select all that apply.)
1. An “effect” is the specific reason for the failure. (False. An effect” is the potential consequence or impact of the failure to the system or end user.)
2. An “effect” is the potential consequence or impact of the failure to the system or end user. (True)
3. An “effect” is the manner in which an item does not accomplish its intended functions. (False. This describes a failure mode, not an effect.)
4. None of the above. (False)
[The intermediate problem repeats the scenario from last month. This month, readers will be asked to continue the analysis by identification of an effect.]
Scenario: On June 12, 1972 an American Airlines DC-10 aircraft lost its aft cargo door soon after taking off from Detroit. We’ll use this incident to practice identifying the elements of an FMEA. Here is the background to the FMEA problem.
McDonnell Douglas learned from cabin pressure testing that an improperly closed cargo door could burst open due to loss of cabin pressure, potentially resulting in the floor of the passenger compartment crashing down into the cargo compartment. The temporary solution was to put a vent flap in the door that would close by the same linkage that shut the cargo door, which would keep the airliner from holding pressure unless the cargo door was safely latched, thereby alerting the pilot to the problem. However, a bit of excessive force by a baggage handler shutting the door could make the vent flap close even though the cargo door was not fully latched.
The DC-10 with the cargo door vent flap was put back in service. On a brief layover before the Flight 96 leg to Detroit, a cargo handler had trouble shutting the rear cargo door, but managed to get it shut with a little extra force. Since the door-latch signaled “closed,” the warning light in the cockpit did not show a problem. However, the force the cargo handler used to shut the door bent a metal linkage on the inside of the door, preventing it from closing properly. The air pressure during ascent generated too much force on the bent door linkage. It sheared off the pins, releasing the door. The cabin near the door collapsed and jammed the control cables to the tail. The rest is tragic history.
The probable failure sequence of the DC-10 cargo door is:
- Airline cargo handler uses extra force to close rear door, bending door pin. Door does not securely close.
- The door vent flap does not trigger the electronic alarm, and the pilot is not notified the cargo door failed to lock securely.
- The air pressure outside the cargo door drops during ascent, until pressure on the door from the inside causes the door-latch pin to shear. The cargo door blows out.
- High-pressure air inside the cabin collapses the floor, resulting in hydraulic lines and cables becoming non-functional.
We’ll use the door latch-pin failure on DC cargo door latching subsystem as an example to practice identifying functions, failure modes, effects, causes and controls, based on the cargo door latch-pin failure history.
In previous months, we focused on one possible function, and one possible failure mode for the identified function. The previous month’s answer for the function of the door latch-pin was something similar to “fully secure the cargo door in the closed position during all operating loads and environmental conditions without allowing the door to close unless fully latched,” and for the failure mode, it was something similar to “Door latch pin bends under maximum stress loading.”
This month we’ll focus on an effect for the door latch-pin of the DC-10 cargo door. We’ll continue this exercise in the next several months as we focus on the proper identification of failure causes and controls.
Problem: Use the door latch-pin failure of the DC-10 cargo door latching subsystem as an example to identify one possible effect for the identified failure mode “Door latch pin bends under maximum stress loading. Try to trace the effect from the local (cargo door) all the way to the end effect on the aircraft system.
For the effect, your answer should be something like “Bent latch pin allows the door to appear to be closed when it is not fully secure, thus failing to abort airplane takeoff, potentially creating a pressure differential between inside and outside air, with the possibility of catastrophic cargo door blowout during flight.”
In this next FMEA Q and A article, a reader asks a challenging question about the linkage between FMEA boundary diagrams and Design FMEAs. There are many linkages between different elements of FMEAs, and this question/answer highlights one of the key linkages.
Ask a question about FMEA
In the words of Albert Einstein, “The important thing is never to stop questioning.” I invite you to ask anything at all about the broad subject of FMEAs. You may be curious about an application issue, or want another view on a challenge you are experiencing. Questions and answers are the lifeblood of learning, and we’re are all learning. I will answer all questions to the best of my ability, and promise to keep personal information confidential.