Phantom Figures: The crash of Tuninter flight 1153

Survivors cling to the wreckage of flight 1153 after the crash. Image source: Bari Today

On the 6th of August 2005, an ATR-72 carrying vacationers to the Tunisian island of Djerba unexpectedly lost power on both engines while flying over the Mediterranean Sea. After trying and failing to relight the engines, the pilots were forced to ditch the plane off the coast of Sicily, resulting in a catastrophic breakup of the aircraft that killed 16 of the 39 people on board. Italian investigators working to solve the case soon found that there was nothing wrong with the engines: rather, the plane had run out of fuel half way through the flight. The inexplicable lack of fuel turned out to be the culmination of a lengthy comedy of errors that started in a maintenance workshop the previous day and quickly spiraled out of control. After unraveling a trail of clues involving a wide cast of characters, investigators were able to reveal a bizarre chain of escalating misunderstandings, misinterpretations, and missed opportunities that led to the flight leaving Italy without enough fuel to reach its destination.

A sister ship of TS-LBB, the ATR-72 involved in the accident. Image source: Roberto Binetti

Tuninter — known today as Tunisair Express — is a small regional airline operating domestic flights within Tunisia as well as to neighbouring countries, including Italy, France, and Malta. Since its founding in 1991, the core of Tuninter’s fleet has long consisted of French-made Avions de Transport Régional ATR-72 and ATR-42 twin turboprop airliners with room for 70 and 48 passengers respectively. Tuninter found it easy to operate both models, which have a common type rating — allowing pilots to fly either one with minimal additional training — as well as a wide variety of interchangeable components, which brought down maintenance costs.

On the 5th of August 2005, Tuninter’s ATR-72 with the registration TS-LBB flew a series of flights between destinations in Tunisia and Italy under the command of Captain Chafik Al Gharbi, one of the airline’s most experienced pilots. During the course of these flights, he observed that some of the lights in the fuel quantity indicator weren’t working, making it difficult to read how much fuel was in the right wing tank. After arriving back in Tunis at the end of the day, he made note of the issue in the aircraft log book so that maintenance technicians could fix it overnight.

A photo of the original FQI on TS-LBB shows that four lights were burnt out, making it difficult to read. Image source: the ANSV

That night, a Tuninter maintenance technician set about replacing the faulty fuel quantity indicator, or FQI. To see if they had any replacement FQIs in stock, he searched the computerized parts catalog provided by the manufacturer and found three different ATR-72 FQIs, with the part numbers 748–681–2, 749–160, and 749–759. But when he checked to see if any were in stock, the results came back negative — not because the airline didn’t have any, but because they hadn’t been entered into the system correctly. The technician decided to widen his search criteria to see if there were any other FQI models available, entering “748-” to see the full range of parts beginning with that number. This turned up another FQI with the part number 748–465–5AB. The information page for this part said it was suitable for both ATR-72 and ATR-42 aircraft. This was in fact false; the part was meant only for the ATR-42. But it also wasn’t in stock, so the technician kept searching. On the same information page, he noticed that part number 748–465–5AB was listed as interchangeable with another FQI, part number 749–158. Since this part was interchangeable with a part that was labelled as suitable for the ATR-72, he assumed it would work, even though it was in fact intended for use only on the ATR-42. He went to the stockroom, checked out a 749–158 fuel quantity indicator, and then clocked out at the end of his shift, leaving the job of actually installing the part to a different technician.

The information page seen by the maintenance technician that led him to mistakenly pick out an ATR-42 FQI. Image source: the ANSV

The technician working the next shift picked up the FQI and set about installing it on TS-LBB. Although the indicator was meant for an ATR-42, it was the exact same size and shape as the indicator on an ATR-72, and it fit in place perfectly. The only visible differences between the parts were the labels over the displays, which read “L.TK:2250” on the ATR-42 and “L.TK:2500” on the ATR-72. Before installing the part, the technician was supposed to cross-check that it was appropriate for the aircraft, but he didn’t do this — in all likelihood, he’d never been given the wrong part before, and had consequently grown lax about checking. Had he taken the time to look closer, he might have noticed that the label was wrong, but he didn’t. He installed the FQI, logged the job as complete, and went on to the next task. No further inspection of his work was required; nor was a dip-stick test that could have ensured that the fuel tanks actually contained the amount of fuel displayed on the indicator.

A comparison of an ATR-72 and ATR-42 FQI. Can you spot the differences? Image source: the ANSV

While an ATR-42 indicator is physically almost identical to an ATR-72 indicator, they are not interchangeable. The ATR-42 has smaller fuel tanks than the ATR-72, and the two types of FQIs consequently use different algorithms to calculate total fuel quantity based on the sensor readings. When an ATR-42 FQI was installed on an ATR-72, it applied the wrong formulas to the source data and produced an incorrect reading. It would indicate 1,800 kilograms of fuel when the tanks were completely empty, and would then add about three kilograms to that reading for every two kilograms that were actually in the tanks. When the plane went in for the repair, it had 790 kilograms of fuel on board, and when it emerged, this had magically increased to 3,100 kilograms without any fuel being added!

Comparison of the two indicators with 400kg of fuel in each tank. Image source: the ANSV

The following day, TS-LBB was scheduled to fly from Tunis to Djerba and back before heading to Bari, Italy to pick up a tour group and bring them to Djerba as well. When the crew for the Tunis-Djerba trip arrived on the plane, the fuel quantity indicator showed 3,100 kilograms — far more than the 1,400 kilograms they would need for the flight. Because carrying extra fuel increases the plane’s weight and decreases efficiency, the Tuninter flight dispatcher informed them that they would have to remove some fuel before leaving the airport. But at that moment the fuel removal vehicle was unavailable. To avoid a delay, the dispatcher switched the flight to a different ATR-72 that had just arrived at the airport.

Some hours later, the dispatcher assigned TS-LBB to operate a short round trip flight to Palermo on the island of Sicily. But the captain of that flight had a grudge against TS-LBB due to a loud and annoying noise generated by its nose wheel while steering on the ground, and he refused to fly that aircraft until it was fixed. There wasn’t time to solve the problem on the spot, so the dispatcher once again switched the flight to a different airplane, and TS-LBB continued to sit on the ground in Tunis. Its next scheduled flight was now the trip to Bari and on to Djerba, which just so happened to be its longest flight of the day.

A map of TS-LBB’s planned route to Djerba via Bari. Map source: Google

In command of this flight was the same Captain Chafik Al Gharbi who had flown the plane the previous day. Assisting him in the cockpit was First Officer Ali Kebaier Al-Aswad, and a maintenance engineer rode along in the passenger cabin in case the plane needed service on the ground in Bari. After boarding the plane, the pilots calculated that they would need 3,800 kilograms of fuel for the round trip flight, 700 more than was indicated on the FQI. A fuel truck hooked up to the plane and added fuel until the FQI read 3,800 kilograms, at which point the tanker shut off the flow and the operator gave them the refueling slip. Had they examined the receipt carefully, they would have noticed that while the indicated fuel quantity went up by 700 kilograms, the truck had in fact only added 465 kilograms of fuel.

A mathematical analysis shows how much fuel the faulty indicator will show, vs the actual amount of fuel. Image source: the ANSV

Shortly thereafter, Captain Gharbi noticed something odd while examining the previous entry in the flight log. After the plane’s last flight, the fuel quantity had been logged as 790 kilograms, but when they arrived that morning, the FQI showed 3,100 kilograms. So where did the extra fuel come from? Captain Gharbi assumed that someone added fuel while the plane was on the ground in Tunis, so he searched for the associated refueling slip, but was unable to find it because it didn’t exist. He then called the dispatcher and asked where the slip was. The dispatcher called the ramp supervisor, who said he wasn’t aware of any fuel having been added. The dispatcher told Gharbi that the most likely explanation was that one of the pilots from the earlier flights that switched to different aircraft had accidentally taken the slip with them. He added that he would look for the slip and would give it to Gharbi when he returned to Tunis.

Although it was against the rules to take off without documentation for all the fuel added since the previous flight, Gharbi was satisfied with the dispatcher’s promise to find the slip, and the plane left Tunis on time for the ferry flight to Bari. With only the pilots, the flight attendants, and the engineer on board, the flight proceeded normally and landed in Bari sometime after 1:00 p.m. But after arriving, Gharbi was faced with another confounding fuel mystery: he had expected to arrive with 2,700 kilograms of fuel left (1,100 consumed), but the FQI only showed 2,300 kilograms (1,500 consumed). A backup instrument — the Fuel Used Indicator, which was independent of the FQI — would have told him that they had consumed 950 kilograms of fuel over the course of the flight, not 1,500; however he didn’t consult it, or if he did, he failed to notice the discrepancy. Instead, he decided that he would add 400 kilograms in Bari to make up the difference. Once again, the fuel truck added fuel until the FQI read the desired amount, and then the operator gave Gharbi the refueling slip. And just like last time, nobody noticed that while the indicated quantity went up by 400 kilograms, the refueling slip showed that only 265 kilograms had been added.

In Bari, 35 passengers boarded the plane, including the maintenance engineer (who was not technically part of the crew). With 39 people on board, TS-LBB left Bari as Tuninter flight 1153, headed southwest across the Mediterranean to the resort island of Djerba. No one knew that the ATR-72 had only 570 kilograms of fuel on board, barely half the amount needed for the journey.

At first, all appeared normal as the flight crossed the Italian peninsula and proceeded out over the Tyrrhenian Sea. But as the plane ran through fuel at a rate of ten kilograms per minute, it soon started to run low. A low fuel warning should have sounded in the cockpit, but it was dependent on the faulty fuel quantity indicating system and consequently never went off. Instead, as the right wing tank began to run dry, the pilots received a low fuel feed pressure warning. This warning could illuminate as a result of fuel contamination, a fuel leak, or a malfunction of the fuel feed system, in addition to insufficient fuel. First Officer Al-Aswad started running through the associated checklist while a perplexed Captain Gharbi tried to figure out what was causing the warning. Realizing that fuel was apparently not reaching the right engine, and that the ATR-72 wouldn’t be able to maintain its cruising altitude of 23,000 feet on a single engine, he called air traffic control and requested a lower altitude due to unspecified technical problems.

Location of the emergency. Map source: Google

Shortly thereafter, the situation escalated rapidly. Before Al-Aswad even made it to the first item on the low feed pressure checklist, the right engine ran out of fuel in its associated wing tank and stopped generating power. Gharbi immediately ordered Al-Aswad to abandon the previous checklist and start the single engine flameout procedure, at which point both pilots set about trying to relight the engine. The first item on the engine flameout checklist was to check the fuel level — but the FQI claimed they still had more than 1,800 kilograms. So they set about trying to restart the engine, unaware that it could not possibly run.

Less than two minutes later, the left wing tank also ran dry, and the left engine sputtered and died. With no engines running, the plane immediately lost electrical power, and the instrument panel blinked and went dark. Suddenly faced with a dire emergency, Gharbi and Al-Aswad kept desperately trying to relight the engines, convinced that they still had 1,800 kilograms of fuel remaining. Al-Aswad declared an emergency and was directed to contact controllers in Palermo on the island of Sicily. Gharbi planned to make an emergency landing at Punta Raisi Airport in Palermo, but they were still more than 90 kilometers away from the airport and falling fast. Their plane was nothing more than a giant glider, a lump of metal with good aerodynamic characteristics and a set of small, difficult to read standby instruments. Already, Gharbi had doubts about making the airport.

As flight 1153 continued to fall from the sky, the pace of activity in the cockpit became downright frenetic. Like a machine, First Officer Al-Aswad responded to a continuous stream of orders, rattling off radio calls, checklist items, and steps on the engine relight procedure. But no matter what they did, the engines wouldn’t restart. Desperate, the crew asked if there was a closer airport than Palermo. There was not.

The pilots soon called the on-board maintenance engineer up to the cockpit to try to help diagnose the problem, but he was just as baffled as Gharbi and Al-Aswad. He couldn’t think of anything that hadn’t already been tried. Facing the unthinkable, Gharbi ordered the flight attendants to prepare for a ditching on the open ocean. Falling through 4,000 feet with 37 kilometers still between him and the airport, he radioed Palermo and told the controller that they were not going to make it, requesting that rescue vehicles — some “helicopters or something” — come out to intercept the plane.

Abandoning their attempts to relight the engines, the pilots switched their full attention to the imminent ditching. Landing a plane on the water is extremely difficult, especially on the ocean, where waves can cause the catastrophic breakup of the aircraft on impact. Good ditching procedure calls for pilots to land parallel to the line of swells, but this is easier said than done; in practice, it’s often impossible to detect which way the swells are moving. But Gharbi did make another excellent decision: spying two large boats off to his left, he told Palermo that he would try to land near the boats, and requested that the controller contact them to inform them of the situation.

Inside the cabin, passengers started to panic. Everyone had put on their life vests, but some inflated them immediately, against the flight attendants’ instructions. Although one flight attendant was proactive about helping, the other had succumbed to panic and became completely unable to perform her duties. Her distress amplified the fear of the passengers, who had not been told what was wrong with the plane.

The pilots would not have the luxury of life vests; they had far too much on their plate. Steering toward the boats, Gharbi slowed down and raised the nose, coming in at the optimal angle of attack to make the landing as smooth as possible. Unfortunately, it was not possible to discern any pattern to the swells on the ocean surface; they would just have to hope the plane stayed intact.

A simulation of the impact. Video source: Mayday (Cineflix)

Moments later, the ditching began. The tail of the ATR-72 struck the water first, bending the fuselage and snapping the plane in half just behind the wings. The front of the plane heeled over and dug into the water at high speed, breaking off the cockpit and throwing passengers out of the aircraft. The ATR-72 came to a stop partially submerged and broken into three sections. The impact instantly killed a number of people and severely injured many more, some of whom quickly drowned after suffering debilitating injuries. Most of the surviving passengers found that they had been ejected from the plane, and those who inflated their life vests before impact were left helpless because their vests had been ripped away during the crash. In the cockpit, the maintenance technician was dead, but Gharbi and Al-Aswad both survived. Gharbi found himself outside the plane, while Al-Aswad regained consciousness inside the submerged cockpit and managed to swim out to safety. All told, 16 people lost their lives in the crash, including the technician, a flight attendant, and 14 passengers, while 23 survived. But the ordeal wasn’t over: stranded far out to sea, rescue would not come quickly.

Survivors cling to the wreckage of flight 1153 after the crash. Image source: Bari Today

The survivors soon gathered around the center section of the plane, which stayed afloat thanks to the buoyancy provided by the empty fuel tanks. Only the wings remained above the surface, and some passengers climbed up on top of them while others clung to the edges for dear life. A few more had been swept farther afield, including one who didn’t have a life jacket and was left treading water.

Unfortunately, none of the nearby boats were aware of the crash, and none of them came to rescue the survivors. A commercial flight taking off from Palermo was asked to fly over flight 1153’s last reported location, and its crew were able to confirm the presence of floating wreckage, but couldn’t tell if there were any survivors. Boats and helicopters were soon dispatched to the scene. By the time the first boat arrived 46 minutes after the crash, the tail section and the cockpit had sunk, but mercifully the wings had not. The patrol boat immediately began plucking survivors off the floating center section, while a helicopter arriving from Palermo threw down life preservers and hauled up the isolated passengers who had drifted away from the aircraft. Amazingly, no one drowned waiting for rescue, and the operation successfully retrieved all 23 survivors.

A map of the locations of the survivors and the fatalities. Image source: the ANSV

Later that day, the still-floating center section was raised from the sea and brought to the wharf in Palermo, where one of the first things Italian investigators noticed was a conspicuous lack of fuel. In fact, had there been an appreciable amount of fuel in the tanks, the wings wouldn’t have floated. But the full story behind the missing fuel only emerged after the recovery of the black boxes and the cockpit instrument panel, which proved beyond doubt that a fuel quantity indicator from an ATR-42 had been installed on an ATR-72. The National Flight Safety Agency (ANSV) was forced to ask: how could such a thing happen?

The sequence of events began in the maintenance facility when the technician began his search for a new FQI. He used the computerized database incorrectly — instead of entering a known part number and determining whether it was in stock, technicians had been using it to conduct more general searches to find out what parts were applicable to the task. The catalog was not designed with this purpose in mind, and using it this way proved dangerous, as minor inaccuracies in the database misled mechanics about which parts could be put on which planes — information they were supposed to already know. This attitude toward the parts database would turn out to be only the tip of a very large iceberg of unsafe practices.

The center section of the ATR-72 is brought to the port of Palermo after its recovery from the sea. Image source: Il Giornale

In fact, the series of misinterpretations and misunderstandings that led to the accident was symptomatic of a generally lax safety culture at Tuninter. The airline was not in the habit of updating the online catalog to reflect what parts it had in stock, effectively rendering the system useless. Maintenance Technicians weren’t adequately checking what parts they were putting onto planes, and weren’t filling out the appropriate paperwork. Many of the technicians didn’t understand the documentation they were supposed to be using. The airline had no quality assurance program or safety management system, which was not required in Tunisia. In hindsight, it was no surprise that Tuninter eventually had a serious incident as a result of improper maintenance.

The same problems extended to the pilots and ground personnel. The dispatcher and Captain Gharbi both agreed to take off without a refueling slip, in violation of regulations. And on the flight out to Bari, the pilots neglected to perform fuel burn checks, which would have entailed examination of the fuel used indicator, allowing them to notice the discrepancy between that instrument and the FQI. Uniting all of these problems was a culture at Tuninter that always assumed everything was running smoothly. When checks turned out fine 99.99% of the time, people simply stopped doing them, allowing a dangerous error to slip through defense mechanisms designed to catch and prevent it.

The aircraft’s intact tail section is recovered from the sea and placed on the deck of a ship. Image source: the ANSV

The ANSV also examined whether flight 1153 could have made it to an airport and came to a surprising conclusion: in theory, they should have been able to reach Palermo. In a simulator, two sets of experienced ATR-72 pilots were faced with the same situation, after having been briefed beforehand. One of the crews managed to land safely in Palermo after gliding almost 100 kilometers, while the other ditched just short of the airport. So why did flight 1153 crash at sea more than 30 kilometers from the nearest runway? As it turned out, it was just possible to reach the airport if the optimal glide speed and angle were maintained throughout the descent — a tall order even in a simulator, let alone in real life. Gharbi and Al-Aswad never consulted the dual engine flameout checklist, which would have described the optimal glide parameters; consequently, they flew too fast, and failed to feather the windmilling propellers, which would have reduced drag. Investigators felt that it was unreasonable to expect the pilots to have reached Palermo under the circumstances, considering that they were faced with a chaotic emergency situation for which they had not been trained. Furthermore, the loss of electrical power knocked out their primary instruments, making it almost impossible to maintain a consistent airspeed and descent rate. They therefore concluded that the simulator run was useful as a proof of concept, but not as a tool for evaluating the pilots’ actual performance. In fact, investigators praised the pilots for conducting the ditching at the optimal pitch angle and speed, ensuring a generally survivable outcome despite difficult water conditions.

The tail section of TS-LBB lies in a field where it was moved for temporary storage. Image source: the ANSV

After the accident, Tuninter hired foreign experts to help it overhaul its safety regime, which included launching a quality assurance program, giving human factors training to maintenance and administrative personnel, introducing periodic refresher training for mechanics, creating new technical logs for pilots, and appointing an employee whose sole duty is to keep the parts database up to date. The ANSV was sufficiently impressed with Tuninter’s advancements that they made no further recommendations to the airline in their final report. However, plenty of other safety issues outside Tuninter needed to be addressed. The ANSV recommended that all ATR-72s and ATR-42s be inspected to ensure that the correct fuel gauges were installed, and that ATR redesign the FQI so that it was physically impossible to install an ATR-42 indicator on an ATR-72 or vice versa. In 2006, a mismatched FQI was discovered on an ATR-72 in Germany after a pilot noticed a fuel quantity discrepancy while parked at the gate; after this incident, ATR agreed to redesign the indicators. Today, the ATR-42 indicator is shaped differently from the ATR-72 indicator and won’t fit into the slot in the instrument panel.

The ANSV also recommended that the European Aviation Safety Agency (EASA) require airliners to have a low fuel warning that is independent of the fuel quantity indicating system; that ditching procedures be revised to take into account a possible absence of engine power; and that a broader effort be undertaken to ensure that physically similar but functionally different components be impossible to swap mistakenly, among other points. To Tunisian authorities, the ANSV recommended that cabin crew be tested on their ability to stay calm in emergency situations, and that every Tunisian airline have a flight data analysis program to monitor aircraft parameters in real time.

The cockpit suffered additional damage during impact with the seabed and during the recovery process; immediately after impact, it remained mostly intact, allowing the pilots to survive. Image source: the ANSV

The crash of Tuninter flight 1153 left behind a legacy of genuine safety improvements, both for Tunisian aviation and for anyone who flies on ATR aircraft. But it also serves as a reminder of how frequent small lapses can eventually turn into a fatal accident. At so many points, the crash could have been prevented. Every one of those forks in the road had to go a certain way for the accident to occur, making the final outcome wildly improbable — and yet, it still happened. Consider that if the fuel removal truck had been available before the scheduled Tunis-Djerba flight, the tanks would have run dry before the gauge reached the desired 1,400 kilograms, and the problem would have been discovered! It was only by sheer coincidence that TS-LBB’s first flight after the FQI replacement was long enough to mask the issue. It also goes to show that people are not good at analyzing the root causes of unusual problems. Despite noticing several discrepancies involving the plane’s fuel over the course of the day, it never occurred to Gharbi and Al-Aswad that the fuel gauge itself might be faulty. Only after the crash, with plenty of time in hospital to reflect, could they have connected the dots. One final lesson therefore might be to pay attention to gut feelings: if something feels wrong, but you aren’t sure what, it’s worth taking a moment to look a little deeper.

A woman examines the centre fuselage section, with seats still attached, after its recovery. Image source: Alessandro Fucarini

There is one final addendum to the story. After the crash, both pilots were arrested, along with several other Tuninter employees, and charged in Italy with failing to ensure the safety of the flight and its passengers. In 2009, a court in Palermo sentenced both Gharbi and Al-Aswad to 10 years in prison. 8–10 year sentences were handed down to the other defendants in the case as well. Specifically, the court accused the pilots of failing to follow emergency procedures after the dual engine flameout, instead “panicking and praying” as the plane fell from the sky — a decision that can only be a considered a gross perversion of justice. While the pilots made mistakes, it is ridiculous to sentence them to prison for expressing panic in an unprecedented emergency, especially considering that a better outcome was unlikely. After air disasters caused by human errors, it can be tempting to point fingers, to try to punish someone, in order to “avenge” the deaths of so many people. But this behavior in fact degrades the safety of the aviation system. When professionals fear prosecution for unintentional errors, they will stonewall investigators out of a sense of self-preservation, refusing to admit mistakes that could serve as important safety lessons. Furthermore, in the absence of deliberate recklessness or gross negligence, making an air disaster a criminal matter achieves nothing. A pilot involved in an accident should not be treated like a murderer and locked away; after all, he didn’t come in to work that day intending to hurt people. For a man who was not a danger to society, the knowledge that he was in part responsible for the deaths of 16 people should have been punishment enough. This ruling was hardly surprising, however: in Italy, such sentences are par for the course. More recently, Italy sentenced several scientists to prison for failing to predict deadly earthquakes, an even more egregious judicial overreach than that suffered by the Tuninter pilots. While I normally refrain from inserting my personal opinions into my articles, I feel that it’s important to call out this mentality for its destructive nature.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.

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