Thursday, January 14, 2010

Master Caution

Taking a little break from the seaplane adventure. On my last 737 flight, we were climbing through about 16,000 feet on the way up to FL410. It was a cloudy and cold day on the ground, about 3 degrees C at the surface, with a broken cloud layer from 4,000 to 8,000, and clear above. During the preflight walkaround inspection, we found a little bit of frost on the top portions of the engine nacelles that were still in the shade. The portions of the airplane in the intermittent sun, including the wing, were all clear, so we chose not to de-ice. Everything else on this glistening, low-time Boeing jet looked perfect. It was my leg so I made the takeoff, and as we climbed through 16,000 feet, the master caution lights suddenly illuminated, along with the ANTI-ICE light on my side.

Before continuing, a little background on alerting systems: The 737 was designed in the 1960s. As such, its alerting system is nowhere near as sophisticated as today's modern airliners with centralized displays that display text messages indicating system failures. Boeing calls this system EICAS, or Engine Indication and Crew Alerting System. Airbus calls this ECAM, Electronic Aircraft Centralized Monitoring. With these systems, when something fails, a master caution/warning light illuminates in front of each of the pilots, and a text message (EICAS message or ECAM message) describing the failure is shown on one of the forward displays. They simplify the alerting because they provide a single place to look to determine what systems are degraded or failed on the airplane. They also provide clear guidance to the pilots about which checklist is appropriate. The crew only has to look up the EICAS message in their Quick Reference Handbook (QRH) to find the correct checklist.

The 737 however, has a system that has been jokingly referred to as "distributed EICAS". This is because lights that describe the failures are scattered all around the flight deck, mostly on the overhead panel. It still uses the master caution lights on the glareshield in front of each of the pilots to alert them to a problem, but instead of a single display that shows EICAS messages, there are two rectangular annunciator panels, one in front of each pilot, informally referred to as "six-packs", that list general systems. There are six lights on each of the six-packs, one for each of the twelve systems. The six-pack on the Captain's side shows: Flight Controls, IRS, Fuel, Electrical, APU, Overheat/Detector. The six-pack on the First Officer's side shows: Anti-Ice, Hydraulic, Doors, Engine, Overhead, Air Conditioning. The intent of the six-packs is to direct the pilots' attention to the appropriate system on the overhead panel.

So when a hydraulic pump fails, for example, the pilots likely will not notice the hydraulic pump low pressure light illuminated on the overhead. So the Master Caution lights illuminate in front of each of the pilots, and the First Officer will see the Hydraulic light on the six-pack illuminated in front of him. This directs the crew to look up at the hydraulic portion of the overhead panel, where the additional lights indicate the status of the pumps. Sound complicated? It is, sort of, when compared to something like EICAS. If this were an airplane with EICAS (or the Airbus equivalent ECAM), like a 747-400, the crew would have seen the Master Caution lights and an EICAS message on a display that said something like HYD PRESS ENG 1. This is a 747-400 message that tells the crew that the engine #1 hydraulic pump has failed. Very straightforward and simple. There's really no need to even look at the overhead panel to confirm the failure.

When we got the Master Caution lights and the Anti-Ice six-pack light, a quick glance up at the overhead showed the L ALPHA VANE amber light illuminated. This indicated the heat to the left angle of attack sensor had failed. It wasn't going to be an immediate problem since we were climbing in the clear, and would stay out of the clouds until descending through about 8,000 feet on approach. I had the QRH on my side, so I handed it to the Captain so he could read the short checklist for the failure. It just said to avoid icing conditions, otherwise we could get erroneous flight instrument indications. We found that the circuit breaker for the left alpha vane heat had popped. After some discussion amongst ourselves and with an engineer, we decided it was safe to attempt one reset on the circuit breaker. A quick push in, and it popped again immediately. So much for that idea. Feel free to insert the standard joke here about using something to hold the circuit breaker in.

We completed our flight and started getting ready for the approach. The weather was the same as on departure, and there was no real way to completely avoid icing conditions. We decided that we would press on and minimize time in the clouds by descending quickly with the speedbrake once we were in the clouds. As we entered the clouds at around 8,000ft, I called for the engine anti-ice to be turned on. Within 1 minute, we started noticing a small buildup of ice on the windshield wiper and bolt. We had been cleared all the way down to 3,000ft, and I was descending with full speedbrake as planned.

As I watched the ice slowly building up, I paid particular attention to monitoring the flight instruments, comparing mine to the Captain's, to see if there were any discrepancies between them. It was as much for the obvious reason of the safety of flight, but also to satisfy my curiosity. We don't train for a frozen alpha vane in the simulator, and although I have some technical understanding of what SHOULD happen to the flight instruments, I've never actually seen it in person. I guess a small part of me was a little disappointed when we finally broke out of the clouds at 4,000ft with no abnormal indications at all.

I'm not going to wish that that happens again though. I think I'd rather try it in the simulator next time I get a chance.

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