Ramblings about flying for fun and profit.

Tag: Alternate Law

The AF 447 Black Boxes

I’m sure  that by now you have read all about the recovery of the flight recorders from the Air France A-330 crash in the Atlantic in June 2009.  There have been several articles about the information they are releasing from the data analyses.  Here is one from the Wall Street Journal that was published a couple of days ago. Here is one from today on the AvWeb site Most of the information from the latest AvWeb article comes from an update to the investigation published by the BEA (the French Aeronautic Agency doing the investigation). A pdf file of the BEA update is here and on the AvWeb site. Northwest Airlines Airbus A-330 If you read the BEA report and are not familiar with the terminology – PNF refers to ‘Pilot Not Flying’ and PF refers to ‘Pilot Flying’ – a way the cockpit duties are assigned once the aircraft starts the take-off. Captains and co-pilots (First Officers) often trade off duties on consecutive legs of a trip. This time it was the co-pilot’s turn to fly.

If you read the report and have any questions about terms or phrases – leave me a comment and I’ll try to clear up the terminology for you.

Once again – I wasn’t there (obviously), I have no first-hand knowledge of what went on in the cockpit nor do I have access to the actual flight recorder data or cockpit tapes. Even so, I thought I’d make a couple of comments.

In the ‘Final Minutes’ graphic in the Wall Street Journal the note mentions the nose being ‘pushed up’ which confuses me a bit. It was obviously not written by a pilot if they are implying that control inputs were the cause if the increased pitch.

The Airbus A-330 is an ‘electric jet’ –  meaning it is controlled by computers. The controls in the cockpit are not physically connected to anything directly controlling the flight control surfaces nor the engine. Everything is done through position and force sensors. The computers controlling the airplane read the forces and positions from the pilot’s movements of the control stick and power levers. The computers  analyze that information and then decide if the control movements that they imply are within the ‘laws’ programmed within their software. If the computers think the request is ‘legal’ they will move the control surfaces and/or change the power setting to match the request.

In the computer operating mode referred to as ‘Normal Law’ there are extensive safeguards enabled – such as the inability of the pilot to input control movements that will allow the aircraft to stall or overspeed.

Increasing systems failures may degrade the operating mode from ‘Normal Law’ to ‘Alternate Law’ to ‘Direct Law.’ As those systems degrade, so do the protections built into the flight control system that control inputs cannot override. In Alternate Law there are still some protections in place, but they act in a different way – and pilot inputs can override the protections. In Direct Law there are no computer-generated protections in effect. There is then a direct relationship between the cockpit control movements and the flight control surfaces – though still through the computers.

The only time that the aircraft is intentionally operating in  a mode close to Direct Law is during the landing flare (a condition called ‘Landing Mode’) – that is so that the pilot has some semblance of normal feel and control feedback during the landing.

While the plane is cruising along at FL350 it is in Normal Law with all the aerodynamic protections of the system in operation.  If the computers, for whatever reason, were to think that the airplane is  flying too fast – up near it’s maximum mach number at that altitude –  they would take actions to protect the airplane. Those actions could include a reduction in power and/or a pitch up to reduce the speed. If the computers thought the plane was flying too slowly it could increase the power to maximum and try to lower the nose.

With the system degraded to Alternate Law and the autopilot and autothrottles disconnected it is up to the pilot to keep the plane upright. Hand flying a plane in the traffic pattern and hand flying the plane at FL350 are vastly different exercises. Then add to that the lack of tactile feedback from the side stick and you multiply the difficulty. The side sticks physically move very little compared to ‘normal’ aircraft control sticks and they have a centering spring that tends to return it to the neutral position.

The Airbus does not take lightly one pilot trying to help the other one with the controls. There is a light and warning visible on the glareshield when both pilots are making control inputs. You can’t tell what kind of inputs the other pilot is using, only that they have used some force on the stick to move it from it’s center position.  The two pilot’s inputs to the controls are added together and the result is sent to the computers. If both pilots pull the stick half way back the computer assumes that you want full back stick. If one of the pilots pulls full back and the other pushes full forward the sum is zero and the controls don’t move.

Now there are calls being made to change the airline training system to include more hands-on experience. It isn’t just the training system that needs to be changed. There also needs to be a change the jet transport philosophy of flying. When autoflight systems became more reliable and accurate the emphasis changed from hand flying the airplane to managing the autopilot.

I can remember going through training in the first versions of the 747 as a copilot and spending the majority of my simulator sessions hand flying the plane while also handling the radio communications with Air Traffic Control. The captain and flight engineer would be handling the malfunction and checklists. There was no requirement to use the autopilot and as I remember it was implicitly discouraged.

By the time I moved to the left seat of the smaller Airbus A-320, then to the right seats of the more automated B-747-400 and then the Airbus A-330, the emphasis had shifted completely. You were required to use the automation to the fullest extent. My impression was that the automation was so complex and capable that philosophy was that the more you used it the more familiar and comfortable you would be with its operation. But that familiarity comes with a price – the deterioration of hand flying skills.

I’ve stated my feelings about relying on automation in previous posts about being a systems operator rather than a pilot. And it seems that I’m not alone in my feelings based upon this opinion piece from a 747 pilot.

Was the lack of hand flying skills a factor in the Air France accident?  Maybe the final report from BEA will say.


Added 6/1/2011

I didn’t see this particular episode of  ‘The Early Show’ where ‘Sully’ Sullenberger was a guest, but this article on the CBS News site relates the opinions Captain Sullenberger expressed concerning the new information that has been released.

And here is an article from Jetwhine where Rob Mark gives his views on the newly released information.

Airbus Incident in New Orleans

In case you missed it, back in the first week in April an Airbus A-320 took off from New Orleans and then climbing through 4000′ the crew got a message on the Electronic Centralized Aircraft Monitor (ECAM) concerning an Autothrottle system problem and then another saying that there was smoke detected in the Avionics Bay.

UAL A-320 landing at SFO. All sorts of things happened after that, ending in a landing back at the New Orleans airport with the nosewheel exiting the runway surface and the crew and passengers evacuating the aircraft using the emergency slides. No injuries were reported from anyone on board the flight, though I imagine New Orleans had a shortage of clean seat cushions the the rest of the day.

As more information came to light (The NTSB Preliminary Report is located here in pdf format) things started sounding a little ‘unusual’ to me. The report says that the crew lost primary instrumentation and on landing did not have anti-skid braking nor nosewheel steering. An Avweb article about the incident adds that it was the First Officers displays that went blank and also adds that the air-driven emergency generator deployed – that would refer to the RAT or Ram Air Turbine.

An interesting comment from the crew in both the Preliminary Report and the Avweb article is that neither member of the cockpit crew smelled or saw smoke during the incident – nor did any of the cabin crew.

First, let me say that I was not in the airplane, nor do I know the pilots, nor do I have any first-hand knowledge of what happened onboard the flight that day. All I have is what is published in the NTSB report , the Avweb article and my experience flying as an A-320 captain.

Second, let me reiterate that in-flight fires are nothing to mess with. All of our checklists say that if a fire is confirmed and cannot be immediately extinguished, you are to descend and divert immediately.

The A-320 Avionics Smoke detection system has an ionization detector (like you have in your house)  in an air circulation vent line. If the detector senses particles that it thinks are smoke it sets off the warning and generates the Avionics Smoke ECAM message.  There is no check for flames or heat associated with the detector.

Our procedures were to accomplish the checklist that appears on the ECAM screen and then go to the written Cockpit Operating Manual(COM) and see if further steps are required and to see what changes to our procedures will be necessary because of the steps we had taken.

As best as I can tell from the manuals that I have , the ECAM checklist on our aircraft started with “IF PERCEPTIBLE SMOKE…”

Our  COM checklist for Avionics Smoke started with :

If smoke is not perceived (sight or smell):
– Continue to search for signs of Avionics Smoke.
– End.

If smoke is perceived (sight or smell):

If you continue with the Avionics Smoke checklist you are to put on your oxygen masks and start a divert to the nearest suitable airport – then start trying to isolate whatever equipment is causing the problem. If you can identify the specific equipment you are to remove power from it. If the smoke is no longer perceptible after 5 minutes the procedure ends.

If the smoke is still perceptible after 5 minutes it is assumed that you have a real fire in the avionics bay (right below the cockpit) and the procedure gets really serious. You shut off the number one  generator (of two) and turn on the Emergency Electrical Power (extend the RAT).  Once you confirm that the emergency generator is operating you disable the APU (Auxiliary Power Unit) generator and turn off the number 2 generator – that leaves the RAT powering the aircraft electrical system. This emergency generator has a much smaller output than either the aircraft or APU – consequently several normally operating aircraft systems are disabled in order to make sure that essential equipment remains operational.

The result is that the First Officer loses all of his displays and all of the number 2 radios cease operation. The aircraft reverts to what is called Alternate Law, meaning that some of the computer-generated aerodynamic protections are lost. When the landing gear is extended this degrades even farther to Direct Law – you then have a manually-controlled airplane instead of a computer-controlled airplane. You also lose both autopilot systems, the autothrust system, one engine reverser, nosewheel steering and the antiskid function of the braking system.

As a recap, you have a hand-flown, single-pilot airplane. The First Officer has no displays nor access to any radios. With one thrust reverser inoperative and no antiskid system, there is a reduced ability to slow the aircraft on landing and once the rudder loses effectiveness only differential braking can steer the airplane. If the nosewheel is not centered the airplane will go wherever it is pointed as there is no nosewheel steering system.

So, what happened on this flight? I don’t know for sure, but according to the Avweb article, the NTSB is saying that there is no indication of any fire or smoke damage.

Anyway, I thought I’d throw this information out there and let you draw your own conclusions. Some day – maybe in 2-3 years – we’ll get the NTSB final report and see what they think happened.

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