BETWEEN THE CONTROLLER’S LIPS, A CATASTROPHE WAITING TO HAPPEN

The expression ramak kala (‘by a hair’s breadth’) is frequently used by safety investigators when examining incidents and accidents. The word ramak, borrowed from Arabic root rmḳ, originally means ‘the final sign of life before death, a glance.

In Turkish, the well-known proverb “The grasshopper jumps once, twice…” points to the fact that if preventive action is not taken after escaping from serious accidents by a hair’s breadth, eventually a catastrophic accident will occur. Near miss events are investigated, root causes identified, and corrective actions implemented to prevent recurrence. .

The expression “weakest link” became famous when it was used as the title of a quiz show. The strength of a chain is equal only to the strength of its weakest link. This is indeed a very accurate description. When pulling or carrying a heavy load with a chain, it will always break at its weakest link. .

One example of such a weak link can be found in the procedure “RNP Approach with Baro VNAV – down to LNAV/VNAV minima”, which is one of the applications of Performance-Based Navigation (PBN). This is a very sophisticated system that enables passenger and cargo aircraft to conduct non-precision approaches at airports where the ILS system is unavailable, using GPS technology. Its lateral accuracy is ±0.3 NM — in other words, the aircraft can follow its intended course with a lateral error of only 0.3 NM. .

While GPS technology allows for precise lateral tracking, another crucial safety issue is the accurate determination of the aircraft’s vertical position. GPS-derived altitude can often be inaccurate. Instead, the aircraft’s altimeter measures external atmospheric pressure to calculate altitude. During an RNP APCH approach, the airport control tower provides the QNH — the current local atmospheric pressure in millibars. The pilot manually enters this QNH into the aircraft’s system. The aircraft then compares its altimeter reading with the QNH to calculate its precise height above ground level. .

Unfortunately, the weak link in this system is that the QNH information is transmitted verbally by the controller and then manually entered into the aircraft system. A mere 10 millibar under-read by the controller would cause the aircraft to believe it was 280 feet higher than it actually was. .



On 23 May 2022 at 13:40, an Airbus A320 aircraft, registration 9H-EMU, operated by Airhub Airlines of Malta, was approaching Paris Charles de Gaulle Airport (CDG). As the ILS system for the intended runway was unavailable, the crew elected to perform an “RNP Approach with Baro VNAV – down to LNAV/VNAV minima.” The aircraft, arriving from Stockholm, carried 172 passengers, 2 pilots, and 4 cabin crew. .

The French female ATC controller, when providing the QNH in English, gave an incorrect value of 1011 mbar instead of the correct 1001 mbar. The Malta pilots entered the erroneous value into the system. As a result, the aircraft believed itself to be 280 feet higher than it truly was. .

The striking detail is that the same controller, around the same time, provided the correct value (1001 mbar) to two French aircraft in French, while only the Malta aircraft received the erroneous QNH in English. .

As the Malta A320 approached dangerously close to the ground, the pilots initiated a TOGA go-around 1.5 NM from the runway threshold, as they had no visual contact with the runway. The radio altimeter showed that the aircraft had been just 6 feet (less than 2 meters) above the ground at that moment. .

This incident narrowly avoided becoming a CFIT (Controlled Flight Into Terrain) accident — an accident in which an aircraft, under the control of the flight crew, is flown unintentionally into terrain. Such accidents often occur not due to technical failures, but because the pilot is unaware of the true position of the aircraft. .

A well-known example of CFIT is the crash of World Focus Airlines’ MD-88 (registration TC-AKM), operating for Atlasjet on 30 November 2007, which struck a mountain near Keçiborlu, 18 kilometers from Isparta, Turkey. .





Another significant point in the Malta A320 incident is that the Honeywell EGPWS (Enhanced Ground Proximity Warning System) on board did not issue the “TERRAIN AHEAD – PULL UP” warning. The French investigation authority BEA determined that this was not due to malfunction, but because the older-generation model installed on the aircraft was not designed to provide a warning in that specific situation. Later versions of Honeywell’s EGPWS corrected this deficiency. .

Tests using the ACSS T2CAS system’s TAWS module under identical conditions successfully triggered the “TERRAIN AHEAD – PULL UP” alert. Unfortunately, it is estimated that around 1,600 Airbus and Boeing aircraft worldwide are still equipped with older-generation EGPWS units that will not issue such warnings under similar circumstances. .

Replacing these units with newer models is not mandated, as the upgrade would be prohibitively expensive. .

Source: BEA Investigation Report on Serious Incident – 9H-EMU, Malta Airbus A320 at Paris CDG, 23 May 2022