When CAS barks

Bingo’s performance as a reliable warning system pretty much sums up my attitude about the crew advisory systems (CAS) most of us now fly with in turbine airplanes. Certainly, a CAS bark calls our attention to an important change in the operational or system status of our airplane. But not always, and then we’re left to determine if the alert is genuine and how to figure that out quickly.

For example, after several hours at high-altitude cruise in a nearly new Cessna Citation CJ4 I was descending close to the 305-knot airspeed redline. The amber master light flashed, and a quick check of the CAS message showed “flaps fail.” I was indicating more than 100 knots above maximum flap extension speed. Maybe not an axe murderer on the porch, but certainly could be something critical.

I was pretty confident that flaps hadn’t extended because there hadn’t been the slightest pitch change, but the “Emergency/Abnormal Procedures” checklist told us the flaps fail message meant the flaps were not in the same position as the flaps selector handle. The flap handle was, of course, up, and untouched in several hours.

The helpful advice from the checklist is to re-cycle the flaps circuit breaker and see if the CAS message goes away. We did—it didn’t. The only other step in the procedure is to prepare for a flaps-up landing. No mention at all of remaining below maximum flap extension speed. It was left up to me to decide if that was a good idea, which I thought it was, and immediately slowed.

After flying the remainder of the descent at 200 knots indicated, we arrived at the spot where one would normally select approach flaps. I had no idea what would happen when the co-pilot moved the handle, but the fail message went out and the flaps extended normally.

On the ground we tested the flaps through full extension several times and everything was normal. But the Textron Aviation people were able read the data stored in the CAS system and see that the flap position sensor on the right wing had sent the wrong signal.

Ipso facto and the sensor was changed in our hangar by the mobile support guys and all was well. At least for a dozen flights or so. Then it happened again. Another position sensor swap. More normal flights, then same flaps fail message. This time circuit boards in the main CAS system replaced. Same result. Talk about a noisy terrier that wouldn’t stop barking.

Finally, the technicians at the main Citation service center in Wichita stumbled on a cold solder joint in a connector in the right wing by wiggling wires and looking for the fault message. That was the fix and it lasted.

The CJ4 limitations prohibit takeoff with any amber CAS message unless there is a procedure in the minimum equipment list (MEL) to handle the situation. But we had no amber message at takeoff so continuing to fly during the “flaps fail” episodes was legal. And after the first couple instances we realized the flaps really weren’t moving when the “flaps fail” alert appeared, which is not exactly positive training reinforcement. We were simply left on our own to decide the CAS barking was invalid. It was only a squirrel.

But sometimes the CAS system is more reliable and sensitive to failure than what we can observe in actual airplane performance. This time the concerning amber alert was “rudder bias fail.” As many, even most, twin turbine airplanes do, the CJ4 has a system that pushes the rudder in the correct direction when power from one engine decreases. The CJ4 uses high-pressure bleed air from the good engine to push the rudder aiding the pilot effort to keep the airplane straight after an engine failure.

The rudder bias system is important enough that the checklist calls for it to be physically tested before takeoff. The drill is during taxi to push one throttle forward and feel the rudder pedal on that side move forward. That, to me, seems like a positive check of proper operation. But the amber rudder bias message wouldn’t go away even though the system checked properly during taxi.

One would think that the MEL would allow a takeoff if the taxi check of the bias system was good even though there was an amber message. Nope. No mention of the rudder bias failure alert in the MEL. We were stuck with an airplane that physically checked good but CAS said it wasn’t.

This time it was CAS that looked smart. Its sensors were able to detect a small bleed air leak in one of the valves operating the rudder bias system. It recorded which valve sent the message and it was a one-day job for the Textron techs to replace the valve and we were back in business.

Despite my carping and suspicion, I must admit the current CAS systems are an important aid to safety, and a vast improvement over the alert/warning systems we flew with not long ago.

When I began flying jets some 45 years ago, it was common to say in a takeoff brief, “We’ll abort for any red light.” And there were many red lights, some important and others you could deal with later. What dawned on airplane designers and certification authorities is that few maneuvers are riskier than a high-speed rejected takeoff (RTO), or a warning light distracting pilots during the final stages of approach and landing.

The solution was to suppress alerts and warnings during takeoff and initial climb, and during the final phase of approach and landing. It was named “dark cockpit” because the distractions of alerts and warnings were suppressed until pilots had the altitude, or were safety of the runway, to deal with a failure.

An element of dark cockpit is to design CAS that has enough analytical capability to prevent warnings of failures that aren’t urgent unless combined with other problems. For example, a generator failure years ago may have triggered a red light. Generator failures are serious, but not critical as long as the other generator continues to operate so now it takes a dual gen failure to trigger the red warning.

In the CJ4 there are 96 possible amber caution messages in the checklist compared to just 27 red warning messages. A few of the red messages are triggered by single events such as smoke in the baggage compartment or engine failed. But most result from multiple failures such as the loss of both air data computers or attitude heading reference systems. A few are even more complex, such as the failure of an AC-DC converter which is only critical after both generators have quit and both converters have failed leaving no power for the avionics.

Perhaps the most important advance in CAS alerts is keeping track of our performance as pilots. For example if the airplane is not configured for takeoff, or the parking brake is on or partially on, or the control lock engaged, you get a red “no takeoff” warning when you advance the throttles.

Then there are other status alerts to less critical conditions such as having the tail deice boots turned on when you climb into air below the minus-30-degree-Celsius operating limit. Not that I’ve ever done that.

When CAS barks we need awareness, and maybe prompt action. Even a possibly extreme event such as an engine fire warning demands we go through the steps to confirm there’s really a fire before we punch the engine out with the fire warning light. It’s CAS job to bark, but it’s still our job to think before acting.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.