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.
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.