Power plus attitude

8588
image: © Katsuhiko Tokunagadact

By Max Collins

Night over the Pacific. A Falcon 900 returning to Perth from the United States via Honolulu and Townsville.

The aircraft was operated by three pilots acting in rotation, and on the sector from Honolulu to Townsville I was the resting pilot. A flight attendant was also part of the crew.

The only passenger was the company chief executive.

Several hours into the flight, with the aircraft cruising in smooth air, the chief executive and I were both asleep in the cabin when I was aroused by a short series of sharp vibrations similar to the tugging of a fish on a line. Moderate intensity. It stopped as suddenly as it started.

I sat up and the chief executive said to me in an agitated voice: ‘Hey Max, what the ##### was that?’

I said, ‘I don’t know,’ because I didn’t know. ‘But I’ll go and find out.’

As I spoke, there was another short set of vibrations. Then, within a second or two of my leaving the seat the aircraft bunted very positively and I hit my head on the ceiling. A couple more bounces off the ceiling and I reached the cockpit door.

This was no longer a polite enquiry. Alarmed now I said to the captain: ‘What the hell is going on?’ or words to that effect.

His reply: ‘The aeroplane’s stalling and we’ve got to get down.’

I looked at the engine instruments: all three were still sitting at about 96 per cent. The captain’s airspeed indicator was ridiculously low, like about 60 kt—which was about 60 kt below stall speed.

He continued: ‘The autoslats are extending.’ It was these that had caused the vibration.

Stalling? At 96 per cent? In normal unaccelerated level flight? It didn’t make sense.

‘We can’t be stalling,’ I said. ‘Get the nose back up to about two or three degrees nose-up. There’s something wrong with the speed indication.’

He kept the nose low, so I leant over him, removed his hand from the yoke and set the attitude at about three degrees nose up. As I recall, the indicated airspeed was very low, near zero.

I told him to hold that attitude while I found the autoslat circuit breaker, which I pulled out. This would stop the slat extensions.

A check of the overhead panel revealed that a pitot heat circuit breaker had popped. I reset it.

Calling the pilot by name I said, ‘there’s nothing wrong with the aeroplane or the engines. How many times have you said that power plus attitude equals performance?’

The indicated airspeed (IAS) soon returned to normal, but before long the circuit breaker popped again, and once again the IAS bled off.

During all this I do not remember referring to the standby instruments, nor what the co-pilot’s instruments were doing.

During the altitude excursion, we hadn’t made radio calls to any agency; the aircraft was slowly returned to its original flight level using the standby instruments.

We decided to divert the aircraft rather than fly on to Townsville with no IAS or autopilot.

We advised Oakland Centre that we had abnormal instrument indications and were diverting to Nauru. Some time later Oakland Centre advised that Nauru was refusing to accept the aircraft.

Discussion in the cockpit determined that Majuro, in the Marshall Islands, was probably the next best option. We advised Oakland and obtained a clearance direct to Majuro.

During late descent into Majuro the captain’s airspeed returned to normal and the arrival was without any further incident.

On the ground, inspection revealed a circle of ice about 30–40mm radius around the angle-of-attack vane, which was frozen in position. This was evidently the cause of the autoslat operation.

The computer associated with the air data system was reset. The angle-of-attack ice then cleared. The popped circuit breakers were reset and stayed in. The aircraft continued to Townsville without further incident.

I sometimes wonder what might have happened if the pilot had not pulled the nose up to three degrees. Would he have done so if I hadn’t made it to the cockpit?

No doubt a second or two later and the power levers would have been at idle. Analysis in that configuration would have been far more difficult, other than the fact that zero airspeed is an impossibility. It is interesting how the abnormal attracts attention and response, even when the abnormal is wrong.

The lesson from all of this? Power plus attitude equals performance. Never forget this. It’s no mere equation, nor is it a cliché. Whether it’s a Jabiru, an FA-18 or an A380, your combination of power setting and angle of attack will determine what the aircraft does. That’s been true since the Wright brothers. It’s the way aeroplanes fly, and the first thing to remember when your aeroplane appears to start flying strangely.

It’s a formula that could have saved all on board Air France 447 in June 2009—had it been applied.

It is also interesting that when pilots are tested for an instrument rating, they are required to fly the aircraft on ‘limited panel’. I wonder if this embeds an attitude that encourages greater faith on the least reliable instruments, the pitot-static pressure instruments. Failure of the attitude reference system is an almost unheard-of occurrence on glass cockpit aircraft. But loss of the pressure instruments can happen to the best and most reliable systems through causes such as sticky tape left over a static port, or a mud dauber wasp making its home in a pitot tube (both of these were involved in fatal air transport crashes). Maybe there is a case for limited panel to refer to loss of the pressure instruments.

LEAVE A REPLY