AT75 en-route near Campinas, Brazil 2024
AT75 en-route near Campinas, Brazil 2024
On 9 August 2024, an ATR 72-500 at FL170 encountered weather conditions which led to airframe icing. Indicated airspeed decreased quickly without corrective action despite corresponding alerts and almost immediately after an ‘INCREASE SPEED’ alert during a turn, the aircraft stalled eventually entering a flat spin until impacting terrain in a residential area. Impact and a post-crash fire destroyed the aircraft and all occupants were killed. The Investigation is continuing.
Description
On 9 August 2024, an ATR 72-500 (PS-VPB) being operated by Voepass Linhas Aéreas on a scheduled domestic passenger flight from Cascavel to Sao Paulo Guarulhos was in the cruise at FL170 in day IMC and in conditions favouring the occurrence of severe icing when a turn was commenced without any attempt to respond to a rapid decrease in airspeed and the aircraft stalled. Control of the aircraft was lost and not regained and it entered into a flat spin completing five full rotations before terrain impact in a populated area. The aircraft was destroyed by the impact and post crash fire and all occupants - 58 passengers and 4 crew - were killed. Despite the close proximity of persons on the ground to the impact location, none were injured.
Investigation
An Accident Investigation is being carried out by CENIPA, the Brazilian Aeronautical Accident Investigation and Prevention Centre. The CVR and FDR were recovered and their data have since been successfully downloaded and relevant recorded ATC transcripts were obtained.
It was noted that the Captain had a total of 5,248 hours flying experience of which 665 hours were on type and that the Co-pilot Captain had a total of 5,143 hours flying experience of which 3,543 hours were on type.
The Investigation has comprehensively reviewed the weather conditions experienced during the flight and concluded that the high humidity and sub zero air temperature “favoured the occurrence of severe icing” in the airspace where the final stage of the controlled flight occurred. It was also noted that for some time prior to the loss of control the aircraft had been in airspace for which severe icing SIGMETs were current.
The progress of the flight has been documented using the data obtained from the CVR and FDR. Fifteen minutes after takeoff from Cascavel, the propeller anti-icing was selected on and two minutes later, passing FL130 in the climb, the Electronic Ice Detector, which senses conditions which favour the formation of ice, was activated which resulted in a CCAS (Centralised Crew Alert System) ‘ICING’ caution on the ICE DETECT panel and a corresponding single chime. As a result, the airframe de-icing was selected on (which results in the Icing Caution light changing from flashing to continuous). Forty seconds after this, another single chime was audible and resulted in the crew “commenting on the occurrence of an airframe de-icing fault” followed by the airframe de-icing being selected off. FDR data showed a corresponding (to the alerts) temporary reduction in actual airspeed.
Over the next hour, the same caution (ICING) occurred three more times, continuing for just over 2 minutes, 6½ minutes and just under 2 minutes before ceasing. Several ‘CRUISE SPEED LOW’ annunciations (the first of three levels of low speed alerting - see the illustration below) were recorded, too. Airframe de-icing was not selected on at all in this period but after the fourth ‘ICING’ caution, it was only 14 seconds before the next such caution began and continued for almost 4½ minutes. After only 12 seconds, the same caution again restarted after which it remained continuously active. This time, the airframe de-icing was selected on and after a further minute, another ‘CRUISE SPEED LOW’ annunciation occurred at a recorded speed of 191 KCAS. The crew continued ATC and on-flight-deck communications about their destination approach with no action in response to the low speed alert and turned off the airframe de-icing system following an exchange with ATC accepting that they would not be able to start their descent until clear of conflicting traffic on crossing course below.
Twenty seconds after this, with the airspeed at 184 KCAS, a ‘DEGRADED PERFORMANCE’ annunciation occurred as the crew were in further communication with ATC after which the Captain “resumed delivery of the approach brief” with the Co-pilot commenting “a lot of icing”. This was followed by the airframe de-icing being immediately selected on for the third time but with no action to address the continued decline in the airspeed. ATC then instructed the aircraft to fly direct to a point in anticipation of clearance to begin descent in two minutes time and during the turn to this point, at a speed of 169 KCAS, an ‘INCREASE SPEED’ alert was activated followed immediately afterwards by the sound of airframe vibration and activation of the ‘STALL WARNING’. Control of the aircraft was lost and it remained in an abnormal flight attitude until it impacted the ground. The high vertical speed and low horizontal speed resulted in a very localised crash site characterised by concentrated debris.
The Aircraft Performance Monitor (APM) panels alerting to loss of cruise speed due to drag attributable to ice accumulation causing a loss of cruise speed at a constant engine power setting
[Reproduced from the Official Report]
The Investigation has already reviewed the comprehensive airframe ice protection systems which the aircraft was equipped with. In particular it has noted that the ‘Ice Evidence Probe’ (IEP) (illustrated below) clearly visible from the left hand pilot position, provides a visual indication of airframe ice accretion in support of the automated alerting provided by the Electronic Ice Detector located on the underside of the left wing. It has also noted that the IEP was “the principal means for the identification of the ice build-up condition on the aircraft” and that it “was designed to retain ice accumulation while all other surfaces of the aircraft were still ice-free” on the basis that the IEP was “supposed to be the first surface to accumulate ice and the last to become free of it” thus providing a flight crew with:
- Visual information on the onset of ice accumulation; and
- Visual information that the aircraft was free from ice.
It has also been noted that the AFM stated that an aircraft “could only be considered clear of ice when the IEP was completely free of any ice accumulation”.
The IEP as seen from the left-seated pilot position with some ice accretion visible
[Reproduced from the Official Report]
It was noted from the CVR that the crew had agreed during the pre flight preparation that the Icing Speed Bugs on their ASIs (see the illustration below) would be manually set to 165 KIAS, which the Investigation has ascertained was correct for the flight conditions. For each flight, this speed was specified as the minimum manoeuvre speed “minimum speed manoeuvres at low bank , 0° flap and icing conditions”. It was noted that one of the indications of severe airframe icing was the inability to maintain the speed above the Icing Bug Speed + 10 knots - the loss of control occurred 11 knots below this figure.
The required responses to the speed loss indications ‘CRUISE SPEED LOW’, ‘DEGRADED PERFORMANCE’ and ‘INCREASE SPEED’ illustrated above which occurred in the final stages of controlled flight have been reviewed and in summary were found to be as follows:
- CRUISE SPEED LOW requires that icing conditions and speed are monitored.
- DEGRADED PERFORMANCE requires that IAS is maintained above ICING BUG + 10 knots and recommends acceleration to above ICING BUG + 30 knots and that the flight path be amended.
- INCREASE SPEED requires that IAS is maintained above ICING BUG + 30 knots and the SEVERE ICING procedure must be applied. This requires:
The Icing Bug (red) as set on each pilots’ ASI for the accident flight
[Reproduced from the Official Report]
Although establishing the integrity or otherwise of the airframe de-icing system and all the related annunciations is the subject of further investigation, it has been noted in respect of the crew comment on the occurrence of “an airframe de-icing fault” earlier in the flight when the first ICING alert occurred, that there was no recorded unserviceability of this system at flight dispatch.
The Preliminary Report was released on 14 September 2024.
Related Articles
- In-Flight Icing
- Aircraft Ice Protection Systems
- Aircraft and In Flight Icing Risks
- Ice Formation on Aircraft
- Aerodynamic Effects of In-Flight Icing
- Icing Certification
Further Reading
- Transport Canada - Extract from Safety Letter 1/2007: The Adverse Aerodynamic Effects of Inflight Icing
- Getting to grips with Cold Weather Operations, Airbus, 2000
- A Study of U.S. Inflight Icing Accidents and Incidents, 1978 to 2002;
- Inflight Icing Educational Objectives for Air Carrier Pilots.
- Aircraft Icing Handbook, Version 1 by Civil Aviation Authority of New Zealand .
- Aircraft Deicing Glossary, June 2024