Nauru airport
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I was flying a Boeing 737 from Nauru to Samoa. Departure was at 0330 local, planned cruise altitude 33,000, trip distance of 1485 nm and flight time 3.5 hours – all of which was over water. Funafuti atoll airport was at the halfway point with the early part of the flight being OCTA (Outside Controlled Area, meaning the captain is responsible for separating his own aircraft from other aircraft). Included among the 25 passengers was a little old lady who had never flown before.

Tracking 113 from Nauru NDB, we experienced the first signs of engine icing while climbing through 25,000 ft so the engine anti-ice system was switched on. Soon after, we were surprised to hear crossing traffic – a B747 – broadcasting an All Stations call on guard frequency that he had left FL 310 for FL350. His position was 120 DME from Nauru while we were 80 DME and he was crossing our radial. We had no previous knowledge from the local flight service unit of the presence of the 747.

Nauru airport

Nauru airport in the South Pacific.

Shortly after we had exchanged position information with the 747 and passing FL320 in IMC, the cabin altitude warning horn sounded. There had been no discernable pressure changes to our ears and it took 25 seconds of raised eyebrows and “what the hell is that noise?” before we twigged that the pressurisation was playing up.

Initially I thought the noise was an over-speed warning – recollecting an accident to a Boeing 727 in the US that literally spun in from high altitude due to erroneous airspeed indications caused by icing of the pitot systems. After all, we had just moments before experienced engine icing indications. Fortunately the First Officer disagreed with my hasty diagnosis and it was then I noticed the cabin altitude gauge was indicating 13,000 ft and still climbing when it should have been steady at 8000 ft.

From here on it was a comedy of errors on my part. While donning oxygen masks I managed to knock off my spectacles which fell to the floor. The old song “My eyes are dim – I cannot see – I have not brought my specs with me” comes to mind. Then there was a few seconds delay as we both fumbled in the dim light of the cockpit to locate the tiny volume control knobs situated on the overhead loud speakers. We needed these on in order to talk through the oxygen mask intercom system. I then recited aloud the memory items of the Rapid Depressurisation Drill which included directing the First Officer to fully close the main outflow valve switch situated on the pressurization panel over his head. From the left seat it is impossible to read the position of the outflow indicator needle. Finally I made a PA to the passengers warning them of the impending emergency descent.

With throttles closed and speed brake extended, I placed the aircraft into a steep dive exactly as recommended and practiced in the simulator. I was concerned about the well being of the little old lady and not too confident our island air hostesses could handle the automatic dropping of cabin oxygen masks – hence the haste to get to a lower altitude as soon as possible. It was a black night IMC descent and I was totally involved with the handling of the descent leaving the pressurization panel to the First Officer. Without glasses, I was unable to focus easily on the instruments, keeping in mind we were descending in IMC at 6000 feet per minute.

By now my ears were painful from sudden changes in cabin pressure and I handed over control to the First Officer while I groped on the floor for my glasses – finally retrieving them from the base of the control column. He was doing a sterling job so I left the flying to him and now with good eyesight I was able to have a better look at the pressurization panel.

I was still unable to relieve the pressure in my ears and the pain was severe and distracting. A closer look at the cabin altitude needle indicated it was now 3000 ft – yet only a minute earlier it read 13,000 ft. No wonder ears were hurting with an average rate of change of cabin altitude of over 10,000 ft per minute. The closure of the outflow valve from its normal slightly open position to fully closed meant an over-pressure had occurred.

By now we had reached around 20,000 ft and with the cabin altitude under control, I re-engaged the autopilot while we assessed the situation. We were in turbulence from clouds, and as it would have been folly to continue to our destination with a dodgy pressurization system I decided to return to base.

Apart from a few sore ears, the passengers were in good shape, including our little old lady. The passenger oxygen masks did not drop down because the closure of the pressurisation outflow valve had prevented further cabin altitude increase to 14,000ft where mask actuation occurs automatically.

After landing the passengers were told to wait in the airport terminal while the defect was rectified. I was told later that when the air hostesses guided the little old lady down the stairs she said “That was quick – are we in Samoa already?” As this was her first time in an aeroplane she obviously thought this was a normal flight!

After an engineer changed the pressurization controller, we flew to Samoa with no further problems. It transpired the crew who had flown the aircraft before us had reported fluctuating cabin rates of climb and written up the defect after landing. Our paths did not cross that night as they had driven straight to their hotel without talking to us.

Meanwhile their engineer had replaced the offending pressurization controller from local stores and signed off the defect report that by unfortunate chance, happened to be on the very last page of the maintenance sheet. He then knocked off for the night leaving a new maintenance book in the cockpit for us to use. We had only seen the new document, which, of course, showed no defects. As we were to find out, the replacement controller was also defective. Happily, the third controller installed was fine.

Clearly there was no need for the rapid descent. I should have directed the First Officer to first read the Unscheduled Pressurisation Change checklist, rather than the Rapid Depressurisation checklist. This would have allowed a more leisurely series of actions to regain cabin pressure. If that did not work, then a rapid descent would have been the next logical step.

With the benefit of hindsight, I would have done things differently. But the urge to descend quickly to safer oxygen levels was strong; more so because of the perceived danger to the little old lady and others from lack of oxygen if the inexperienced cabin attendants failed to act to ensure all passengers were on oxygen. It would have been wiser to concentrate on proper diagnosis of the problem rather than be distracted by concerns over passenger welfare.

One point should be made, however – and this is not an attempt to gloss over events. During proficiency checks in recurrent simulator training, the syllabus invariably required the instructor to initiate an explosive decompression scenario, requiring swift action to initiate an emergency descent.

In retrospect, it would have been better if crews were given practice at unscheduled pressurisation changes rather than accent emergency descent training. There is documented evidence these are more likely events in real life – as we found out. The initial confusion as to the nature of the cabin altitude warning horn was in later years a causal factor in a Helios B737 accident. In the event, I rushed into the emergency descent, perhaps unconsciously primed by previous simulator training – rather than keeping a cool head and sitting on my hands until proper diagnosis of the problem.

John Laming
6 replies
  1. Vicki Bevan
    Vicki Bevan says:

    You make a good point that training for the most likely emergency may lead to misdiagnosis of a less likely problem. Even when adrenaline is flowing, we must keep evaluating the evidence and guard against the easy or familiar answer.

  2. John Zimmerman
    John Zimmerman says:

    Something like this has been debated with regard to the Colgan Air crash in Buffalo. Was that pilot over-trained on tailplane stalls in icing conditions and overlooked the much more common wing stall? We’ll never know, but it is possible.

  3. Larry D Barr
    Larry D Barr says:

    Captain, I’m not faulting you here for your response to the situation, although I agree with your hindsight conclusions. However, I do have a problem with the fact that you were forced to consider in your response the inexperience of the cabin crew. I believe that those of us who are “sitting in the back” on any given flight deserve to have an experienced cabin crew that does not need to be compensated for in the pilot’s response to a real or perceived emergency. Your flight ended well, but had the situation been more serious, the inexperience of the cabin crew could have resulted in injury or loss of life of passengers. Airline operators should ensure that cabin crew is fully capable of responding to any in-flight situation.

  4. John Laming
    John Laming says:

    Thank you for your well made point re cabin crew. In some third world countries including those Pacific small operators, the cabin crew are usually locals who have priority in jobs over experienced overseas cabin crew. Some may be close relatives of local politicians who ensure through family ties they get employed – regardless of suitability. These crew may be given easy passes into the airline regardless of examination results. Family connections gets you a job. I am sure things have changed for the better over the years since the above story of the emergency descent. To illustrate a little more, I was the captain when a passenger had great difficulty breathing during cruise in the 737. A young CA administered oxygen and when the passenger got worse I was called down the back to see what was going on. A quick check of the frightened passenger revealed the CA had omitted to turn on the valve of the portable oxy bottle and with a plastic mask over her face the poor passenger could not breathe. Of course passengers are entitled to expect experienced CA but experienced doesn’t always mean competentin some parts of the world. Passengers are also entitled to expect two experienced pilots up front. Yet in many big international airlines the first officer (who is the legal second in command) is young and inexperienced having only recently left basic flight school. In fact with the rapid expansion of airlines in Asia and the Middle East it is common to have newly graduated pilots as first officers with less than 500 total flying hours. The passengers are unaware of this because all they see are pilots with lots of gold stripes and smart uniforms.

  5. Roger Ritter
    Roger Ritter says:

    I was on a KC-135 flight (military version of the 707, sort-of) as a Civil Air Patrol officer with eight cadets when the plane suffered a stuck cabin outflow valve. There was no sudden decompression – the cabin altitude just rose with the airplane until it was at 20,000 feet. The flight crew lit the Oxygen sign, and we pulled the masks out of the bags we were issued at the beginning of the flight. The bags had a portable oxygen bottle and mask with 10 minutes supply. After about 5 minutes trying to fix the problem, they gave up and descended to a breathable altitude (12,000 feet, I think) and we returned to base. Two of my cadets got faulty masks, though, and we had to swap them out with on-board spares before the descent. Even though we didn’t get to see any in-flight refueling on that trip, it made for a good story when we got back to our squadron.

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