Tuesday, June 18, 2024

Foreign Object Debris Still A Threat

By Bikram Vohra

During the Farnborough Airshow on 25 July 2000, Air France Flight 4590, a Concorde passenger jet on an international charter flight from Paris to New York, crashed shortly after take-off.

The Smithsonian describes the chilling last seconds of the flight in graphic detail. One of Flight 4590’s Goodyear tires encountered a piece of debris that had escaped the engine cowling of a Continental Airlines DC-10 as it took off just minutes prior. The sharp metal strip made mincemeat of the rubber, which was under considerable strain, supporting the full weight of the notoriously heavy Concorde and its massive fuel supply distributed across seventeen tanks.

A large chunk of tread from the blown-out tire slammed into the underside of the plane’s left wing, sending an internal pressure shock wave propagating through the fuel in one of the primary tanks. When the pressure wave reached a weak point in the tank, it ruptured, causing a gout of atomised, highly flammable fuel to spew out towards the aircraft’s rear. Already past the velocity of no return for take-off, pilot Christian Marty soon received a frightening alert from the control tower: “You have flames behind you.”

90 seconds later, 109 people were dead, and the mystique of Concorde was forever destroyed.

“F-BVFA Concorde Air france Manchester 27-08-1990” by Rob Hodgkins is licensed under CC BY-SA 2.0 DEED

Categorised under the label of Foreign Object Debris (or damage), anger at that spectre has never disappeared. The human factor kicks in, and with aircraft density increasing exponentially, the odds of something being left behind do become a recipe for disaster. 

Add fallibility and Murphy’s law to the mix, and you have a recipe for disaster.

Careless maintenance items left in an aircraft can lead to various safety risks, including potentially dangerous situations for both passengers and crew. These risks can manifest in a range of scenarios, from minor inconveniences to catastrophic failures. Here are some details on how such items can generate risks:

Weight & Balance Issues: Aircraft are meticulously balanced to ensure safe and stable flight. Careless placement of maintenance items can disrupt this balance.

Foreign Object Debris (FOD): Maintenance items left in the aircraft cabin, cockpit, or engine compartments can become foreign object debris. FOD can be ingested by engines, leading to engine damage and potential failure. Ingested FOD can cause compressor stalls and, in extreme cases, complete engine shutdown.

Electrical & Avionics Issues: Maintenance items left in sensitive areas, such as the avionics bay or cockpit, can interfere with aircraft systems and electronic components. Even a loose cup of coffee on the flight deck is a hazard if it spills.

Cabin Safety: Leftover maintenance items in the cabin can pose safety hazards to passengers and cabin crew. Loose objects can become projectiles during turbulence or sudden movements, potentially causing injuries.

Maintenance Access Points: Items left in maintenance access points, such as panels, hatches, or doors, can compromise the structural integrity of the aircraft. Improperly secured panels can detach during flight, leading to increased drag, decreased aerodynamic efficiency, and potential damage to the aircraft’s surface.

Fire Hazards: Depending on the nature of the maintenance items, they might pose fire hazards if they encounter flammable materials or electrical systems. A small oversight, like leaving a tool with a battery in an inappropriate location, could result in a fire that endangers the aircraft and its occupants.

Even mice can be a major issue and are seen as foreign object debris. A rodent on a flight, if not caught, demands a landing.

FOD is not something to take lightly. A FOD walk-in at Gauahati in July 2023 produced bags full of debris, including pebbles, screws, and nuts.

The biggest fear is the twin threat of wrenches and screwdrivers. Let us look at a report of one such incident. On 20 March 2017, during a pre-dawn take-off for a single pilot positioning flight from Boise Air Terminal/Gowen Field (BOI), Idaho, Western Airlines Metroliner suffered substantial foreign object damage due to a forgotten tool.

According to the safety investigation report from the US National Transportation Safety Board (NTSB): …the pilot reported that prior to departing on the repositioning flight, he reviewed the maintenance data [sic] and found everything to be up to date. The subsequent preflight inspections, inclusive of the interior and exterior of the airplane, revealed no anomalies.

The pilot stated that at 0400, he called BOI ground control, obtained taxi clearance, and taxied to runway 10L, where he began the take-off roll. The pilot reported that everything was normal, rotated at 105 knots; shortly thereafter, he heard a “pop”, followed by a vibration. Thinking that he had a blown tire, he waited a few seconds to see if the frequency of the vibration would change as the tire rotation slowed. 

However, the vibration remained the same, which led him to think that there might be an issue with the propeller. The pilot radioed the BOI tower controller, advising him of his intention to return to the airport… The pilot stated that on the downwind leg, he thought there might be a problem with a propeller.

The pilot then landed uneventfully, taxied to parking, and shut the right engine down first. The pilot then shut the left engine down. During the last few rotations of the propeller, he observed the spinner wobble slightly; he then noticed that the tip of one of the blades was missing.

 About four inches (100 mm) of one blade was missing, and there were three holes in the fuselage.

Two pieces of blade debris were found in the aircraft, and the armrest on the jump seat was also damaged.

Airport operations personnel recovered another piece of propeller blade debris from the runway and “what appeared to be the blade of a screwdriver and two pieces of a screwdriver handle.”

Why did it happen? The mechanic was called away suddenly, and, in haste to leave, he ‘left’ the screwdriver behind.

Foreign object debris examples include Tools, parts, and loose hardware. Building materials. Paper, paper clips, pens, coins, and badges. Fragments of broken pavement. Trash, food wrappers and beverage containers. Rocks, sand, and loose vegetation. Baggage tags and pieces of luggage. Hats, rags and gloves, birds, wildlife, and stray animals. Volcanic ash. Humans are not inanimate, and FOD usually harkens to that state, but they do become eligible for this dubious label when they come too close to engines, taxiing aircraft or vehicles airside.

And humans also have frailty. On 3 March 1974, a Turkish Airlines McDonnell Douglas DC-10 crashed shortly after take-off from Esenboğa Airport in Ankara, Turkey. The aircraft, operating as Flight 981, experienced a catastrophic cargo door failure, leading to explosive decompression and structural damage.

This incident highlighted the importance of proper cargo door design and maintenance procedures to prevent structural failures and in-flight emergencies. It was believed that the ground staff had not locked the door properly. That door became a FOD.

In January 2022, the Directorate General of Aeronautical Quality Assurance (DGAQA) in New Delhi brought out a comprehensive 15-page report on the risks of FOD and how to manage it. The fact is that military aircraft, including choppers, are more prone to error from debris than their civilian big brothers.

Representational image: Picture Courtesy/FOD Prevention

As was done in Guwahati, what is needed is more vigilance, extra safety nets and frequent walkabouts by trained personnel. In 2015 Accident investigators in New Zealand reported damage to the flying controls of a 737 due to a trapped cleaning cloth.

During routine maintenance of a Boeing 737-838, engineers found metal filings next to the stabiliser trim cable drum in the forward electronics and equipment compartment (located underneath the flight deck).

While investigating further, they found what they described as a cleaning ‘rag’ had been trapped in the windings on the forward cable drum. Just a small piece of cloth.

An average of 10 FOD incidents a year are reported because there is a noticeable fallout. Where engines are concerned, they fall into four categories of damage and repair.

Minor – no more than blade blending is required.

Moderate –  replacement of blades on a single stage is required 

Severe – replacement of blades on more than on stage is required

Very Severe – blade replacement is required, plus repair of additional damage incurred to the other engine areas.

And while much can be done on the ground, there is truly little you can do with birds. Bird Aircraft Strike Hazard, or BASH, will continue to be a major hassle and is undoubtedly at the centre of the FOD war. 

Bikram Vohra is the Consulting Editor of Indian Aerospace & Defence

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