Most of our abnormal/non-normal training situations present scenarios that are clearly addressed in our manuals. We practice a familiar process:
- Identify the abnormal indication
- Reference the applicable procedure in our abnormal/non-normal procedures manual
- Perform the corrective actions
- Modify our gameplan to accommodate any residual effects from degraded systems
- Choose to continue or land
Occasionally, we encounter anomalous situations that:
- Generate indications and symptoms that are unfamiliar
- Emerge from unknown causes
- Are not covered in our manuals
- Significantly increase situational complexity and event novelty
Following is a situation from a EMB-175 crew (minor changes were made to the following text to improve readability and to clarify the sequence of events – NASA ASRS report 1714483).
We departed normally, climbed to cruising altitude and leveled off. Approximately 6-7 minutes into cruise, a sudden humming noise was heard from an unknown location. It grew gradually into a loud whistling sound with vibration. We were unable to determine source of sound. The Lead Flight Attendant called shortly after inquiring about the noise and informed us that the noise was coming from L1 boarding door. We cross-checked status pages and pressurization. All indications were green and normal. No EICAS MSGs were presented and crew communication was established.
We requested descent from ATC to lower altitude. Center assigned 10,000 feet. The Captain called for the donning of cockpit oxygen masks. Pilot Flying duties were assigned to the First Officer. The Captain attempted to communicate with cabin crew to investigate the noise. The sound was too loud for FA (Flight Attendant) to hear the pilots. We asked ATC about the nearest airport, ZZZ1, at 45 miles. Weather there was ¾ mile visibility and a ceiling of 900 broken. We determined that ZZZ was the most suitable airport and proceeded there. We requested and were given direct to the destination, ZZZ. Center provided priority handling for us. We notified Dispatcher of priority handling and intentions. Our Dispatcher concurred with our decision. We removed oxygen masks at 10,000 feet and proceeded to destination. No limitations were exceeded. No FARs were violated. We requested CFR (Crash Fire Rescue) equipment to be standing by. We taxied off the runway and rescue vehicles were told to stand down.
Surprise, startle and early problem diagnosis: The first effect that the flight crew might have experienced in this situation was surprise or startle. Imagine these two pilots on a routine climbout on a familiar flight. Suddenly, there is a “sudden humming noise was heard from an unknown location” that “grew gradually into a loud whistling sound with vibration”. The startle effect typically degrades our higher-level thinking. We need some time to restore our situational awareness. As we recover, we prioritize our assessment of critical parameters and systems, then work down to lesser parameters and systems. We can imagine this crew quickly glancing at their critical systems (like engines) to see if they had a serious problem. While neither pilot reports their early problem diagnosis steps, whatever they were doing was quickly interrupted by the Lead Flight Attendant calling up to report that the sound was coming from the L1 cabin entry door.
Focused problem diagnosis: The Flight Attendant’s information focused their attention on pressurization concerns. They checked systems and noted, “all indications were green and normal”, and “no EICAS MSGs were presented”. While this may have eased their concerns about critical systems, it may not have decreased their “fear of the unknown”.
Probable cause and early precautionary steps: Those of us with experience flying older airline aircraft recognize this as a faulty door seal event – something that occurred fairly often with older aircraft designs. Typically, the noise started when the pressurization differential between the cabin and the outside air reached a particular threshold. The damaged or misaligned seal began to whistle and vibrate. One other possibility was an unsecured door strap that got caught partially outside the aircraft. At the right speed, it would begin to whip around and beat against the side of the aircraft. In either case, the aircraft typically maintains cabin pressure. The plug door design ensures that it will not blow out or create further hazard. In fairness to this crew, the NASA ASRS report doesn’t record the maintenance diagnosis or remedy – so we don’t know the cause of this event.
Newer aircraft with better-engineered door seals have produced a prevailing environment where most pilots never experience door seal events like this. We can theorize that neither pilot had since the Captain elected to don oxygen masks against a possible rapid depressurization if the door blew out. This was probably unnecessary and perhaps added to the follow-on communications difficulties with the Flight Attendants. Still, in the heat of the moment, it probably felt like a prudent precaution.
Forming a gameplan: The crew elected to make an expeditious recovery and landing. This was the most prudent gameplan to minimize the time exposed to the very loud and unnerving vibration and whistle. There were two landing options. The closest option (ZZZ1) had unfavorable weather. The further option (ZZZ) was apparently better. The Captain divided crew duties by assigning flying duties to the FO while they tried to communicate with the FAs (unsuccessful because of the noise), coordinated priority handling with ATC, informed their Dispatcher, and coordinated with local ATC. They completed their descent to 10,000 feet, removed their oxygen masks, and flew an approach to ZZZ.
Crash Fire Rescue (CFR) response: The Captain requested a CFR response for their landing. Across our industry, there is a pervasive culture of reluctance to “roll the trucks” except for the most obvious or dire emergency situations. Many would judge this Captain’s choice as unnecessary. Given the Captain’s likely mindset, I disagree. Each of this crew’s actions reflected a mindset of guarding against the unknown. Given this, it is both reasonable and prudent to mobilize all assets to favor desirable outcomes. After landing, CFR can provide immediate and detailed information about the security of the L1 door. They can’t do this from the inside of the firehouse, so roll the trucks. Reassured that the door was intact, the Captain chose to release CFR crews.
Summary: In hindsight, we can categorize this as a fairly benign “emergency” event. What made it concerning was the extremely loud and scary noise emanating from an unknown cause. Unknown causes should cue us to select conservative gameplan options – as this flight crew did. The event followed the typical progression:
- Surprise/Startle from an anomalous event
- Initial recovery and critical system assessment
- Crew coordination and problem diagnosis
- Gameplan selection and contingency preparation
- Execution of the gameplan
- Resolution of the event to reach a safe conclusion
Consider other similar scenarios with strong “unknown factors”. In Master Airline Pilot, I present a particularly scary event (Box 22.4: Loud Bang Begins an Unknown Cause Emergency Event – pages 454-455.) In that event, “large sections of air plumbing in the belly of airplane, fed by the Ram Air Inlet, had exploded into pieces.” The highly anomalous event generated particularly worrisome symptoms heard and felt throughout the aircraft and from the flight controls.
Dealing with events with unknown causes and lack of procedural guidance compel us to revert to basic emergency event guidance like MATM:
- Maintain aircraft control.
- Analyze the problem.
- Take appropriate action.
- Maintain situational awareness.
We need to diagnose the problem and form a gameplan concurrently. This usually means that we need to repeat early assessment and later planning steps to continuously validate and update our gameplan. (See Master Airline Pilot, the Risk and Resource Management model (RRM) in chapter 10).
Please feel free to respond with your ideas and impressions.
