Air Asia 8501 Wreckage. (Image: Sky News)
When Jets Get Upset.
By Owen Zupp
The findings into the loss of Air Asia 8501 were released yesterday. As always, the loss of a commercial airliner is a devastating event that the world still struggles to come to terms with. While statistics tell us that such an accident is a rare event in modern aviation, the sight of a charred or floating fuselage is no less distressing. At the core of such devastation, one of the major causes to routinely surface is the ‘loss of control in flight’. So how is it that in the 21st Century an advanced airliner can become out of control?
What is ‘Jet Upset’?
In 1985, China Airlines Flight 006 entered a steep spiral following the shut-down of an outboard engine on the Boeing 747 while cruising at 41,000 feet. With the autopilot engaged, the speed began to bleed off with an increasing degree of asymmetric yaw proving more and more difficult for the autopilot to handle through the use of aileron control alone. Finally, the Captain disconnected the autopilot, but the aircraft subsequently rolled through 60 degrees of bank before pitching nose down dramatically, losing over 10,000 feet in 20 seconds. The severely damaged aircraft was finally recovered around 10,000 feet above the ground. Although some passengers were injured; all survived.
By contrast, the outcome was the loss of all on board in 1991 for the crew and passengers on board a United Airlines Boeing 737 approaching to land at Colorado Springs Airport. The pilots were well aware of the clear air turbulence that could be generated by the prevailing winds, although they could not possibly foresee the dramatic impact it would have upon their relatively routine flight. Only a minute after noting the first fluctuations in airspeed, the aircraft had rolled inverted, pitched its nose down vertically and crashed just 3 miles short of the airfield.
Both the China Airlines and Colorado Springs events are instances of flight control being lost. However, they are just two occurrences in a long line of similar events. In recent times, loss of control has been highlighted in the tragedies of Air France 447 and Air Asia 8501. In fact, in terms of worldwide airline fatalities, the loss of control ranks a close second to controlled flight into terrain (CFIT) in its prevalence. Various studies place the number of lives lost at around 2,000 per decade through this type of accident.
This loss of control in flight is often referred to as a ‘Jet Upset’. Through a variety of possible instigators, the aircraft ultimately entersan extreme or abnormal flight attitude. In an effort to quantify what is an ‘abnormal’ attitude, the following limiting values generally apply;
- Pitch attitude greater than 25 degrees nose up.
- Pitch attitude greater than 10 degrees nose down.
- Bank angle greater than 45 degrees.
While these parameters may be seen as mild for a single-seat Pitts undertaking an aerobatic routine, they are quite abnormal for airline category aeroplanes. By comparison, even when an airliner is conducting a maximum speed emergency descent, it will not exceed 10 degrees nose down and most aircraft will start to bark warnings at the crew if the bank angle exceeds 35 degrees. And yet, throughout history and through a variety of circumstances, jet airliners have continued to experience ‘upsets’.
The Usual Suspects...and others.
Jet upsets are not a common occurrence, but like CFIT, their very nature often equates to a horrendous outcome. Another challenging aspect of jet upsets is that they are so varied in their origins. The causes can be as varied as man, machine and the very environment through which they pass.
Many upsets are environmentally induced. Under the broad title of turbulence, clear air turbulence (CAT), mountain wave effect and windshear are regular offenders. Severe weather phenomena such as thunderstorms, icing and ‘microbursts’ have also lead to bringing down perfectly serviceable aeroplanes. One of the major contributors to jet upset events is ‘wake turbulence’. Although generated by fellow aircraft and not Mother Nature alone, it is definitely a feature of the aviation environment. The spiralling vortices that drift down invisibly from the tips of aircraft are a necessary evil by-product resulting from the generation of lift. Worst at the high angles of attack and slower speeds of approach and landing, they leave following aircraft particularly vulnerable close to the ground. That’s not to say that striking wake turbulence in the thin air of the upper flight levels is not possible, or equally attention-grabbing. Fortunately, through further study, aircraft design features such as winglets and air traffic separation standards, much is taking place to reduce the incidence of wake turbulence events.
Aircraft systems are not without blame when it comes to jet upsets. Generally speaking, aircraft system failures are rare. Even so, flap asymmetry, issues with flight spoilers or an erroneous stall warning may all contribute to a potential upset. Similarly, instrument failures, or conflicting information being presented to the crew have led to aircraft entering abnormal flight attitudes. While this has occurred through misinterpretation and subsequent incorrect inputs by the pilot, auto-flight systems are no guarantee of safety either. Autopilots, autothrottles and automatic stabiliser trim control have all been culprits at one time or another. The reliance upon these normally reliable systems can often lead to a temptation to leave them engaged until they are at the limit of their abilities. In fact, sometimes automation can mask the true cause and reducing the level of automation can assist the pilot before the situation deteriorates too far.
As always, the human factor cannot escape mention. There are rare occurrences of pilot incapacitation, but far more frequently it is the misleading sensory inputs and pilot disorientation that are recognised for their role in the loss of control in flight. However, while a major player, visual cues and vertigo are not the only vulnerable aspects of the human form. The ability to both gather and interpret information from the instruments is a critical pilot skill and varies from one pilot to another. Control inputs are subsequently based upon the timely, correct understanding of the information presented and an incorrect response can not only initiate an upset, but exacerbate it once it has commenced.
More related to our psychology than biology are the potential traps of inattention and distraction. In the recent crash of Turkish Airways 1951 at Amsterdam Airport, a faulty radio altimeter caused the autothrottle to decrease the engine power to idle prematurely during approach. While initiated by a system fault, the investigation was at a loss why the subsequent decreasing airspeed and attitude change was not detected by the crew until the ‘stick-shaker’ was activated due to the impending stall. At times, the early stages of an upset are subtle and will slip beneath the guard of a less-than-vigilant crew, while at other times, the crew will be blatantly distracted by another problem or secondary duties. The old adage of ‘fly the aeroplane first’ never goes out of fashion.
In some instances system faults are overlooked and other times they are initiated by the crew through the inappropriate use of automation. At times the problems stem from a lack of understanding of the system, while on other occasions the usual reliability of the system has led to an underlying level of complacency in its ability. Ironically, the very automation that has been designed to reduce cockpit workload and safety can be the link in the chain that ultimately leads to disaster.
As seen in the Turkish Airlines accident, it is often a combination of more than one element that results in the abnormal flight condition. The aircraft, the crew and the weather may all play their part in the eventual outcome. What is important is that crews are aware of the implications of a ‘jet upset’ and have the skills and presence of mind to suitably recover the aeroplane.
Air France 447 (Image: Daily Mail UK)
Don’t Get Upset.
In the case of ‘jet upsets’, prevention is definitely better than cure. To this end, education has increased significantly over the past 15 years to enhance crew awareness. The continuing education process in certain weather phenomena such as wind-shear and microburst is further assisted by improving detection equipment on the ground and in the aircraft. In the same way, manufacturers are always endeavouring to improve the safety of their product, predominantly for the safety of all on board and partly because a hull loss is the worst publicity an aircraft type can receive.
However, even with improved systems, the human-automation interface will remain an area of ongoing attention. Too often history shows that the aircraft was behaving correctly given the autoflight mode that had been selected. The crew had either selected the wrong mode, or failed to intervene when the aircraft first diverged from what the pilots considered to be the intended flight-path. Even so, jet upsets will still occur and along with theoretical education, pilots are increasingly trained in managing the aircraft once the normal parameters of flight have been exceeded.
Because there is often a conflict between man and automation in upset events, there is a broad philosophy to reduce the level of automation when initiating a recovery. That is to say those items such as the autopilot and autothrottle should be disconnected and the aircraft manually recovered. Removing the automation from between the pilot and the aircraft’s flight-path at this critical stage effectively puts the pilot closer to the core problem. Hopefully this will make the task of interpretation and recovery a more direct series of events.
Jet upset training can relate very closely to the earliest days of pilot training and the recovery from unusual attitudes (U.A.). Now, as then, the resulting flight attitudes and relative states of energy of the aeroplane are virtually limitless in number. As such, the training in recognition and recovery is a thorough process from the classroom to the simulator. Even so, there are some broad principles that are widely recognised.
Firstly, recognise and confirm the situation. The crew should cross-check all the flight instruments in case a faulty dial is about to lead them down the wrong path. Then, as previously stated, reduce the level of automation and fly the aeroplane. Disengage the autothrottle and autopilot and return the ‘feel’ and the ability to respond in a timely manner, back to the pilot.
In all recovery techniques there is an assumption that the aircraft is firstly recovered from the stalled condition. Remember, a stall occurs when the aerofoil has exceeded the critical angle and can occur at absolutely any attitude or airspeed. Aside from the stick shaker, there may be one or a combination of airframe buffeting, a lack of control authority in pitch and/or roll or an inability to arrest a descent rate. If the aircraft is in a stalled state, gravity is flying the aeroplane, not the crew. Get control.
Recovery techniques will vary whether the aircraft is nose high or nose low. Sometimes thrust will be required to restore energy and sometimes it may need to be reduced in an attempt to lower the nose. Roll control in some cases may be needed to return to wings level flight while in others used as a secondary device to establish a nose down pitch rate in a severe nose high situation. The options are almost limitless, hence the training is challenging. Abnormal attitudes call for positive action and at times significant force, but not brutal inputs. A common warning is that the excessive use of pitch trim or rudder may aggravate an upset situation or result in a loss of control or expose the aircraft to high structural loads. This was seen with the loss of an American Airlines Airbus A300 in New York only weeks after the 9/11 attacks. Initially upset by the wake turbulence of a preceding Boeing 747, the pilot’s subsequent rudder inputs resulted in the vertical stabiliser separating entirely from the aeroplane.
An Ongoing Challenge.
With so many potential causes for a jet upset and so many possible methods of recovery, the challenge to conquer this hazard will inevitably be ongoing. Already a great many strides have been made through awareness and training, but complacency has no place in aviation.
The growing levels of automation offer many safety advantages, although the potential to erode manual piloting skills must continually be addressed through continuing simulator proficiency. This is particularly important in terms of jet upset recovery as these core skills will ultimately be required to recover the situation. Across the world, airlines have recognised this fact and routinely train their crews in the techniques to return an aircraft to normal flight from all manner of abnormal situations.
Yet for all the training, constant vigilance and healthy suspicion of the aircraft systems and ambient conditions, pilots will always provide an essential first level of defence. In the ideal world, an aircraft would never need to be recovered from an abnormal situation, but alas, this is not a perfect world. As such, it is imperative that crews are able to recognise the warning signs and be at the ready for those rare instances when jets get upset.