November 2001
During a climb-out over Connecticut skies, a 737 has just ascended to about 16,000 feet when the cabin altitude horn sounded. The pilot reported, "I immediately scanned the instrument panel for some indication of what had caused the horn, no lights were illuminated to indicate the cause for the horn. If a cabin pressure light was incorporated into the master caution system, we could have solved the problem earlier.
November 2001
During a daylight climb-out in U.S. skies, another horn sounded in a 737-300 and the pilot reported, "I did not equate the horn sound with a cabin pressure prob. In fact, my immediate thought was that there was a landing gear malfunction. We scanned the cockpit, did not see any lights or obvious malfunction and I continued flying while the captain continued searching for the prob. At some point, the master caution illuminated, the overhead light illuminated and we saw the passenger oxygen mask light on. We saw that the cabin alt was too high. I did not equate the horn when it went off to a pressurization prob."
October 2004
Cruising at altitude, a 737-300 experienced problems when the auto throttle disengaged and would not work again. "The next thing the crew heard was an intermittent horn sounding in the cockpit which we interpreted to be the tkof (takeoff) warning horn," the pilot reported. "In our minds we were dealing with an air/ground sensing prob and the tkof warning horn sounding would not have been an unusual thing to happen in that situation. After approx a few minutes the first officer remembered that the tkof warning horn also doubles as the cabin alt warning horn. He checked the cabin altimeter and saw that it was above 10,000 ft. This is when we realized the cabin was slowly losing pressure."
The pilot added, "A safety issue I would like to raise awareness about based on my experience is the lack of wisdom in having the tkof warning horn double as the alt warning horn. Because the cabin was losing pressure slowly, we did not feel any pressure changes in our ears that would have normally served to alert us to a pressurization prob. If the first officer had not happened to remember that the horn also serves as a cabin alt warning horn, we may have continued trying to troubleshoot the air/ground prob until passing out from lack of oxygen. Flight crews are not accustomed to associating an intermittent horn with the cabin alt warning system. During the capt's cockpit setup the intermittent horn is tested by advancing a throttle to check the tkof warning system. Like Pavlov's dogs, this forms the habit pattern of only associating that sound with that system. There is no test performed on the cabin alt warning system and as a result there is no particular sound flight crews are trained to associate with that system. Hearing the intermittent horn only served to reinforce in our minds that we are dealing with an air/ground sensing, tkof warning horn prob and distracted us from the more serious prob of losing cabin pressure. Something as important as the cabin alt should have a warning system with its own unique sound that cannot be associated with any other prob. I would suggest a voice that says 'cabin alt.' Pilots can be trained that hearing that sound only means one thing, 'put your mask on now!' "
November 2005
During a climb-out, a 737-800 experienced some turbulence, when the warning horn sounded. The pilot report said it took the cabin crew half a minute "to identify the warning horn's cause," saying it was the first time the crew members heard the horn in more than 20 years of flying.
February 2006
During a dawn departure climb-out, the crew of a 737-400 heard the warning horn at 12,000 feet. "We did not recognize it for that at the time," the pilot reported. "We looked around for any other indications and thought that maybe it was the autopilot. We disconnected the autoplt and we then got a master caution light. I then noticed that the cabin indicator was indicating above 10,000 ft and said we have a pressurization problem. I should have recognized the alt warning horn earlier. I also called for the wrong checklist and we should have had our oxygen masks on. Horn sounded familiar to tkof warning or autoplt warning with no associated panel or overhead lights on."
February 2006
At cruise altitude, a 737-400 captain heard the warning horn. "I commanded the first officer to silence the horn. I considered reaching for my oxygen mask but decided it was not necessary because the cabin was, at all times, below 10,000 ft. I should have put on my oxygen mask the moment I heard the cabin alt warning horn. It is the first action item."
February 2006
At cruise altitude, another 737-400 pilot noticed his plane was descending somewhat rapidly -- with no warning lights. The aircraft, he was to find out later, had a history of pressurization problems that had set off the warning horn.
The pilot wrote, "Given the development and increasing sophistication of aircraft systems on the B737 and its variants, it is noteworthy that the cabin alt warning system (an intermittent aural horn) has remained virtually unchanged for nearly 50 years and shares similarities with the B727. Why this system has not been tied into the master caution warning system escapes me. Pilots have had 'recognition' problems with this singular warning. Pilots now have to 'interpret' what the horn means. In numerous instances, the pilot response is, 'why is the tkof warning horn sounding?' interestingly, on the ground, our training response to this horn is 'do not tkof' or more simply, 'do nothing' which is similar to what some pilot responses are inflight. Some of these pilots have recently made the transition from other aircraft types that have 'voice' warnings, further complicating recognition of a high cabin alt. Lastly, in instances where the insidious onset of hypoxia may have been a factor, a singular aural warning may be nearly impossible for a confused hypoxic pilot to properly interpret and respond with the proper procedure. Recommendation: upgrade the warning system to include a new rectangular amber warning light ('cabin alt') in close proximity to the 'horn silence' button on the overhead panel and the cabin pressurization indicators. These lights should illuminate shortly before activation of the cabin alt warning horn. Others may have an even better suggestion than the one I have provided, but this seems to be a reasonable, relatively inexpensive and simple modification."
July 2006
Reaching a climb-out altitude of about 16,000 feet, the captain and first officer of a 737-400 were surprised to hear the warning horn. "Neither of us had any ear distress that would indicate a pressurization change," the pilot wrote. This, the pilot said, "indicated that the prob was most likely the horn itself as there was no loss of pressure. We decided that the best course of action was to leave the pressurization controls in their present position and simply return to for corrective maintenance."
