Checks and Balances

At the behest of Greek Air Accident Investigation and Aviation Safety Board (AAIASB), aircraft manufacturer Boeing has put out an all-operator message reminding B737 aircrews that the aural warning for an improper takeoff configuration (e.g., no flaps) is the same alert that sounds when the cabin altitude exceeds 10,000 feet. The AAIASB is investigating the fatal crash Aug. 14 of a Helios Airways B737-300 near Athens (see ASW, Aug. 29). Incomplete pressurization, and the reasons therefore, are the focus of the Greek investigation, and crew confusion over the cabin altitude warning system is one of the issues under scrutiny. The Boeing message reminds aircrews of the dual functioning of the aural alert, reminding that the takeoff configuration warning only occurs on the ground, and that the cabin altitude alert sounds while airborne.

On some other aircraft, a two-step cabin altitude warning is provided:

Step 1: Caution. At 8,000 feet, the cabin altitude master caution tone sounds, with an amber light (as opposed to the advisory green light on the B737), or its electronic flight information (EFIS) equivalent.

Step 2: Warning. At 10,000 feet the cabin altitude master warning tone with a red light annunciates, and is repeated again if the cabin altitude warning has been silenced and the cabin altitude is still above 10,000 feet.

We understand that there is no Step 1 on the B737. Also, on Step 2, Boeing's tone is also used for the takeoff configuration warning and no red light illuminates.

The absence of Step 1 drives the crew directly into an emergency situation that requires the donning of oxygen masks and initiating descent; that is a reportable incident with the potential to escalate into a dangerous unpressurized climb well above 10,000 feet.

With the 8,000-foot caution, there is plenty of time for the crew to switch on the bleed air before 10,000 feet.

The pressurization panel itself is checked during the preflight scan, normally by the first officer but hopefully also by the captain (however his across-the-cockpit view of it is very oblique). Significantly, there is no specific challenge and response for the setting of the pressurization mode in any of the ensuing checklists (Before Start, After Start, and After Takeoff). Consider:

• The Before Start checklist would include the response: "1 Pack, Bleeds on, set" to the "Air conditioning and pressurization" challenge. The word "set" is key. What is set? The planned flight level. Yes. The Land Alt? Yes. The pressurization mode? Yes. Here's the problem: there is one challenge and response that relates to the pressurization mode selection, but it is not specific on particular switch selections. Rather, it relates to the way the pressurization panel as a whole is set.
• In the After Start checklist for aircraft with a digital system, there are no challenge and response items that relate to the pressurization panel. Air conditioning panel? Yes. The air conditioning packs are set to AUTO and the Isolation Valve is AUTO. But not the pressurization panel.
• In the After Takeoff checklist, again there is no specific challenge and response to the pressurization mode selection. It's a general check. Packs are AUTO, bleeds are ON.

There have been several reports of B737s taking off with the bleeds off. There are probably a like number of non-reported cases. Clearly, it is time to review the adequacy of the cabin altitude warning system, and related checklists.