"When Jets Get Upset." (Part Two)

CLICK HERE for 'Part One'.

The Usual Suspects....and others. (continued)

....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.

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.

                                "When Jets Get Upset."

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 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 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 enters an 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.

Even so, it is possible for an aircraft to be ‘upset’ within these parameters if it is flying at speeds inappropriate with the conditions.

While these parameters may be seen as mild for a 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..........

Check back fpr Part Two of "When Jets Get Upset". You may also be interested in 'The Fatal Stall'. CLICK HERE.

                    "Traffic Ahead." A very popular image on the blog this past week.

Hi All,

Thanks once again for a tremendous week at this aviation blog. Your support just keeps on growing!

The choice of highlighting the most popular blogs is getting more difficult each week as the numbers seem to be rising right across the range of stories. In a news sense, the post on the collapse of 'Air Australia' seemed to strike a nerve. As a series, 'The Practical Pilot' seems to be very popular, so you can be sure that more of that style of content will be on its way. Similarly, the second instalment of 'The Fatal Stall' inspired quite an amount of comment and feedback; particularly given the fate of Air France 447 in more recent times.

I would like to humbly thank Karlene Pettit for mentioning this blog and profiling me at her very popular website. I also received my copy of her book, "Flight for Control", in the mail a couple of days ago and I recommend that you check it out at her website also.

Episode 82 from the lads at the PCDU podcast hit the airwaves where we chatted about topics like 'The Fatal Stall' and the recent Air Test of the GippsAero GA8 for 'Australian Aviation' magazine.

Well, you've probably grasped that it's been another busy week and I'm currently organising content for the seven days ahead. It's a significant task, but the ongoing support of this aviation blog is making it all worthwhile.

Please keep the feedback and comments coming and don't forget to subscribe to this website or 'Like' me on Facebook.

Cheers for now,

Owen

Those who have been following the blog will recall the chilling video that formed the nucleus of the post, 'The Fatal Stall'. In this instance, the aircraft involved was a light aircraft, but in recent times the tragic demise of Air France 447 has reinforced that size is not a barrier to the lethal nature of the stall. And yet, despite the publicity, Air France 447 was not the only airliner to fall victim to the edge of the aerodynamic envelope. Below is the story of another.

                          "Even the Mighty Can Fall"

The Boeing 727 is one of the all-time classic airliners. Built for speed, it is a pilot’s aeroplane that offers a hushed ride for its passengers by virtue of its three aft-mounted Pratt and Whitney engines which left most of its noise in its wake. Yet even such an illustrious machine can fall victim to the simplest oversight as the crew of Northwest Orient Flight 6231 learnt on a cold winter’s night in 1974.

Prelude to disaster.

The sector should have been little more than a milk run. The flight was a short positioning sector from New York’s JFK Airport to the upstate town of Buffalo where an American football team and its staff were awaiting a ride back home. The crew of three were the only occupants for the ferry flight on the evening of December 1st and consisted of the Captain, First Officer and Flight Engineer. The Captain had held a command for five years, while the F/E had around 2,000 hours in the back seat. The First Officer was to fly the sector and had previously been a Flight Engineer before changing to a ‘window seat’. Of his 1,500 hours as a pilot, only 50 of them were on the 727.

As the crew readied the Boeing, the forecast for the night ahead was typical of a cold winter’s eve on the east coast. The cloud base sitting at around 5,000 feet with occasional thunderstorms extending towards 30,000 feet and icing virtually assured for all levels in between. The crew had planned to cruise above the weather at 31,000 feet and had very little reason to believe that the flight would be anything other than routine. And yet in less than 15 minutes after departing JFK they would crash to earth in a forest a mere twenty miles to the north. But how?

The Tragedy.

Air Traffic Control’s first indication that a problem existed for Flight NW6231 came in the form of a Mayday call stating that they were, “...out of control and descending through 20,000 feet”. In response to ATC’s transmission, the crew’s final message was that they were, “...descending through 12....we’re in a stall.” In less than a minute they were dead.

The Boeing had hit the earth at high speed and its wreckage was confined to an area less than 50 metres square. There were some tailplane components a short distance away, but it was evident that these had separated in flight due to high aerodynamic loads. The undercarriage was retracted, but the leading edge devices were extended. While a number of the aircraft’s pitot heads had been damaged, two of the airspeed sensing systems were found to contain water and began to point the finger at possible icing issues. This was further reinforced when closer examination of the flight deck found the overhead ‘pitot heat’ switches were in the ‘OFF’ position.

Much more would be revealed when the aircraft’s Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) were analysed. As the flight was so short, both recorders contained voice and data recordings from before take-off to right up until impact and both portrayed a relatively normal flight prior to 16,000 feet. At this point the aircraft was established in the climb at 300 knots and climbing at 2,500 fpm enroute to cruise at FL310. Incredibly, the airspeed began to climb through 340 knots while the climb rate exceeded 5,000 fpm. The First Officer was taken aback by the increase; however the crew put it down to the aircraft being light and possibly an updraught associated with the forecast thunderstorms.

Incredibly, as they climbed the indicated performance was astronomical with a speed of more than 400 knots and 6,000 fpm rate of climb as the aircraft punched through 20,000 feet. Still hand flying the aeroplane the First Officer was continuing to pull back on the control column in a vain attempt to arrest the growing airspeed as the overspeed warning sounded. The Captain encouraged the new co-pilot to keep pulling back but it was all to no avail as the overspeed warning sounded again. And then, only seconds later, the stick-shaker activated to warn of an impending stall.

On the verge of 25,000 feet, the crew were still convinced that they were flying at excessive speed and rationalised that the stick-shaker was in fact Mach buffet at the other end of the performance envelope. The Captain again told his co-pilot to “Pull it up”. The gear warning horn chimed in, indicating that the gear was retracted while thrust levers were closed and idling at the stops. It was now all too much for the 727 and the airliner lurched from its nose high attitude to more than twenty degrees down as it simultaneously turned to the right through 180 degrees to point back in the direction of JFK. In an instant its rate of descent increased to 15,000 feet per minute. Yes, 15,000!

The aircraft continued its downward plunge and at around 12,000 feet the Captain recognised that the aircraft was stalled and called for “Flaps Two” as the stall warning continued intermittently. Ultimately the aircraft was descending with a 50 degree pitch down, 80 degree roll to starboard and a rate of descent of up to 18,000 feet per minute in association with G-Forces reaching 5G. Ironically. By this stage the airspeed indicator was reading zero as the tailplane partially failed under the load. The crew never stood a chance at this point and their fall from over 24,000 feet had taken a minute and twenty seconds.

The Findings.

The attitude indicators were frozen at 20 degrees nose down at the point of impact, but it was what they were indicating a few minutes earlier that was crucially overlooked. Climbing through 16,000 feet when the incredible climb performance first began to accelerate, the pilot’s action was to increase the back pressure in an attempt to arrest the blistering speed. If due attention had been paid to the aircraft’s attitude, the nose was actually more than 30 degrees up, when a more likely attitude was in the realm of five degrees. Continuing to pull back only exacerbated the issue until the critical angle was exceeded and the aircraft stalled. So began the aircraft’s rapid fall to earth, but even so, there was no attempt to roll off the high bank angle as the aircraft descended so that any back pressure on the control column merely served to increase the G-loading. Everything was working against the crew possibly recovering the aeroplane.

But why would the crew receive such phenomenally high airspeed indications when in fact the aircraft was stalling with a high nose attitude and the thrust levers were closed? The simple answer is that the pitot heat had not been selected prior to take-off and the multiple probes had iced up until they were blocked passing 16,000 feet. The CVR revealed that there had been some hesitation, confusion and oversight when the pre-take-off checklist had been read, with the ultimate result that the pitot heat was not selected ‘on’. Such a simple error, but such an extreme result. The indicated airspeed was far from accurate once the system was blocked to the extent that when the aircraft stalled at 24,000 feet it was indicating over 400 knots but flying at less than 170 knots. Conversely, in the final stages of descent when there was zero indicated airspeed, the aircraft was probably flying in excess of 350 knots. Without due attention to attitude, confusion undoubtedly reigned supreme.

The Lessons Learnt.

For such a tragic outcome, the findings revealed that it was the failure to successfully complete a checklist that created the problem and an undue focus on airspeed rather than aircraft attitude that led to disaster.

The need for checklist discipline, whether for the lone pilot or the airline crew, is absolutely vital. Cockpit interruptions are frequent and distracting, but shouldn’t circumvent crucial checklists. If the flow of a checklist breaks down for any reason, there is a strong case to go back to the beginning and start it again. And then, don’t stow the checklist, move the marker or flip the page until the checklist is absolutely completed. Even if flying solo, recognise this point by stating out aloud, “Pre-take-off checklist complete.” It provides a further filter and is another marker in the brain that the job is done properly.

As for flying an aircraft with suspect instrument indications, the first thing is to consider whether the attitude and thrust setting is appropriate for the indicated performance. Confirm that the pitot or probe heat is selected to ‘ON’ and cross-check the Mach/airspeed indicators against each other as the fault may not lie across the entire system. First and foremost, consider the attitude and thrust in light of the performance.

The investigators did not find any great mystery in the loss of Northwest 6231. A simple oversight here and a misinterpretation there led to the catastrophic loss of an essentially serviceable Boeing 727 and its precious crew. Regardless of the size of the aircraft or the experience of the crew, the operation of any aeroplane is hinged upon discipline and the observation of some fundamental principles. This is sometimes easier said than done, so it is our responsibility as aviators to give due attention to every aspect of our operation and guard against the curve-balls that fate throws at us.

That cold night over New York the unfortunate crew saw events unfold at a rate that exceeded the ability to recognise what was befalling them and it all stemmed from failing to flick a switch. We are all only human and there but for the grace of God go I.

The full NTSB Accident Report for Flight 6231.

Title Image: r2suberti.blogspot.com

B727 Image. The extensive 'Ed Coates Collection'