investigation report · year of manufacture 2000 total flying time, hours 8,349.73 flying time...

INVESTIGATION REPORT

BEECHJET 400A CABIN DECOMPRESSION ON 02.01.2016

TALLINN 2016

ESIB: A020116 EECAIRS EE001/020116/SCF-NP

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Contents Synopsis ............................................................................................................................................... 3

1 Factual information ..................................................................................................................... 3

1.1 History of the flight .............................................................................................................. 3

1.2 Injuries to persons ............................................................................................................... 4

1.3 Damage to aircraft ............................................................................................................... 4

1.4 Other damage ...................................................................................................................... 4

1.5 Personnel information ......................................................................................................... 4

1.6 Aircraft information ................................................................................................................... 5

1.7 Meteorological information .................................................................................................... 10

1.8 Aids to navigation .................................................................................................................... 10

1.9 Communications ...................................................................................................................... 10

1.10 Aerodrome information ........................................................................................................ 10

1.11 Flight recorders...................................................................................................................... 10

1.12 Wreckage and impact information ........................................................................................ 10

1.13 Medical and pathological information .................................................................................. 10

1.14 Fire ......................................................................................................................................... 11

1.15 Survival aspects ..................................................................................................................... 11

1.16 Tests and research ................................................................................................................. 11

1.17 Organizational and management information ...................................................................... 12

2 Analysis ..................................................................................................................................... 13

2.2 Technical aspects ..................................................................................................................... 13

3 Conclusion ................................................................................................................................ 14

(a) Findings .............................................................................................................................. 14

(b) Causal factors .................................................................................................................... 14

(c) Contributory factors .......................................................................................................... 14

4 Safety Recommendations .......................................................................................................... 14

4.1 Actions taken so far .......................................................................................................... 14


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Abbreviations used in this report

ACC Area Control Center

ATPL Airline Transport Pilots Licence

CVR Cockpit Voice Recorder

DC Direct Current

EASA European Aviation Safety Agency

EETN Tallinn Airport

ESIB Estonian Safety Investigation Bureau

EU European Union

FDR Flight Data Recorder

FL Flight Level

HF PRSOV High Flow Pressure Regulating Shut-Off Valve

HP High Pressure

ICAO International Civil Aviation Organisation

LF PRSOV Low Flow Pressure Regulating Shut-Off Valve

LH Left Hand

RH Right Hand

STC Supplemental Type Certificate

TC Type Certificate

UTC Coordinated Universal Time


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Date and time: 02.01.2016 at 08:20 UTC

Place of serious incident: Estonia

Registered operator: Fort Aero AS

Aircraft Type: Hawker Beechcraft Beechjet 400A

State of registry: Estonia

Registration no: ES-NXT

Synopsis On 02.01.2016 at 21:09 local time, a Beechcraft Corporation Beechjet 400A departed from Pulkovo

Airport Russia with only captain and co-pilot on board. Due to icing weather conditions during level

flight at FL300, the aircraft had wing, stabiliser and engine de-icing systems activated. Since the wing

leading edge was free of any visual ice accumulation and icing was not indicated by the wing icing

detecting system, the captain decided to switch off wing and stabiliser de-icing system. The flight crew

noticed a slight pressure change in the cockpit just after switching off the system, but considered it to

be normal for this type of aircraft. Shortly after, the flight crew got “CABIN PRESS LO” warning and

after a few seconds identified that the cabin pressure was slightly below 10000 ft and was reducing

4000ft/min. Both crew members donned their oxygen masks and started emergency decent. After

switching off Engine Anti-Ice during the descent, airflow to the cabin was restored. The aircraft landed

safely at EETN Tallinn Airport at 20:42 local time. None of the crew members got injured in this serious

incident.

Estonian Safety Investigation Bureau opened an investigation to determine the causes of the serious

incident. The investigation was conducted according to the ICAO Annex 13 and EU Regulation

996/2010.

The investigation determined as the probable cause of this serious incident being the malfunction of

high flow pressure regulating shut-off valves.

1 Factual information

1.1 History of the flight

On 02.01.2016 a Beechcraft Corporation Beechjet 400A aeroplane, registered as ES-NXT and operated

by Fort Aero AS was heading from Pulkovo Airport, Russia, to Tallinn Airport, Estonia. The aircraft

departed at 21:09local time with only two crew members – a captain and a first officer on board. After

reaching cruising altitude 30 000 feet (FL300), the aircraft was flying in clouds. Due to icing weather

conditions the aircraft had wing, stabiliser and engine de-icing systems switched on. Since the wing


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leading edge was free of any visual ice accumulation and icing was not indicated by the wing icing

detecting system, the captain decided to switch off wing and stabiliser de-icing system. Just after

switching off the system, the flight crew noticed a slight pressure change in the cockpit, but considered

it to be normal for aircraft with this type of bleed air system configuration. While the captain (acting

as pilot in command) switched off the de-icing system, the flight crew was handed over to Tallinn ACC.

As they started to make radio contact with Tallinn tower, the flight crew got “CABIN PRESS LO” warning

and after a few seconds identified that the cabin pressure was slightly below 10000 ft and was reducing

4000 ft/min. Both crew members donned their oxygen masks and started emergency decent. The

captain reported emergency to Tallinn ACC and the aircraft was cleared to descend to 2500 ft. During

descent the flight crew went through cabin decompression and emergency descent procedures, and

by turning off the engine anti-ice, airflow to the cabin was restored. The aircraft landed safely at EETN

Tallinn Airport at 20:42 local time. None of the crew members got injured in this serious incident.

1.2 Injuries to persons

No injuries.

1.3 Damage to aircraft

No damage.

1.4 Other damage

No damage.

1.5 Personnel information

Captain

Male, 35, holding ATPL(A) license and I class Medical Certificate. Last proficiency check 07.11.2014.

Experience, hours: Last 90 days Total

On Beechjet 400A 40 615

All types 69 5730

First officer

Male, 27, holding ATPL(A) license and I class Medical Certificate. Last proficiency check 05.12.2014.

Experience, hours: Last 90 days Total

On Beechjet 400A 64 624

All types 64 848

The crew was well rested and the Flight and Duty Time requirements were met.


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1.6 Aircraft information

ES-NXT, Nextant 400XT, is a Beechcraft Corporation Beechjet 400A aeroplane with Nextant Aerospace

L.L.C modifications (EASA STC 10042091 and STC 10042353) incorporated into the aircraft. The

modifications replaced the original Pratt & Whitney JT15D-5 turbofan engines with Williams

International FJ44-3AP engines and installed Rockwell Collins Pro Line 21 Electronic Flight Instrument

System (EFIS) with Rockwell Collins FMS-6100, Localizer performance with Vertical Guidance (LPV)

approach capability and Universal Systems Company (UASC) Terrain awareness warning System

(TAWS).

Aircraft ES-NXT

TC-holder

STC-holder

Beechcraft Corporation

Nextant Aerospace L.L.C

Type Beechjet 400A

Serial number RK-268

Year of manufacture 2000

Total flying time, hours 8,349.73

Flying time since latest inspection, hours 55

Number of landings 6,685

Certificate of Airworthiness issued on 24.03.2014

Airworthiness Review Certificate date of expiry 23.03.2016

Engine

TC-holder Williams International

Type FJ44-3AP

Number of engines 2

Engine Left Right

Serial no. 252771 25772

Number of cycles since latest overhaul 631 631

The FJ44-3AP turbofan engines are enclosed in nacelles and are mounted to the airframe on pylons, one on each side of the aft fuselage. With the installation of Williams International FJ44-3AP engines several other modifications have been made to the aircraft: - Full Authority Digital Engine Control (FADEC) system is installed; - new engine cowlings are installed; - engine pylons are reshaped; - switch and indication changes are made in the cockpit; - wiring changes are made to support the modifications;

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/df21fb0cd091c33d86257bde00559d21/$FILE/A16SW_Rev_28.pdf


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- The Wing Anti-Ice and Inlet Anti-Ice systems has been modified.

The one difference between Pratt & Whitney and Williams engines is that the later produces higher pressure and temperature bleed air than the original engines. The highest temperature can be over 500°F and pressure over 300 psig. The pneumatic system has been modified to reduce the temperature and pressure of the bleed air. Aircraft engine bleed air is mainly used to pressurize and heat the cabin and to provide hot air for

anti-icing system. The system consists of a bleed air cooling heat exchanger, a diverter valve, a

high flow pressure regulating shut-off valve (HF PRSOV), a low flow pressure regulating shut-off

valve (LF PRSOV) and an Inlet Anti-Ice valve (Figure 1, Figure 2).

To reduce the temperature of the bleed air provided by the Williams engines, a bleed air precooler assembly is installed on the engines. The bleed air precooler assembly consist of coils of finned tubing contained within a housing located at the aft bypass duct (Figure 1). To control the bleed air temperature, a bleed air diverter valve is mounted on the lower aft side of the engine (Figure 1). Bleed air diverter valve controls the flow of bleed air through the heat exchanger. The diverter valve does not completely shut off the flow in any direction. When the diverter valve is diverting the maximum amount to the heat exchanger, there is still a small amount of air flowing direct and bypassing the heat exchanger. When the diverter valve largely bypassing the heat exchanger, a small amount of air will still flow through the heat exchanger.


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Figure 1. Bleed air system components (Nextant Aerospace Instructions For Continued Airworthiness)

The diverter valve is driven by an electric motor that is installed on the side of the valve. A control box, located in the aft compartment of the aircraft, receives inputs from various aircraft systems, and from a temperature probe installed in a duct downstream of the diverter valve. The control box uses these inputs to determine how to position the vane in the diverter valve to achieve the preset temperature. The diverter valve attempts to regulate the bleed air temperature to one of two set points. In normal operation, with the bleed air only being used to pressurize the cabin, the diverter valve will attempt to regulate the temperature to 500°F (216°C). If the anti-ice systems are being used (engine or engine and wing) the regulation point will increase to 600°F (315.5°C). The diverter valve can only move to its maximum positions, therefore at coldest outside air temperatures the supplied bleed air will simply match the engine bleed temperature. Likewise in very hot conditions the diverter valve control box will move the diverter valve to the maximum cold position where it does not regulate the temperature but simply move the valve to the coldest position possible. After passing through the bleed air diverter valve and precooler assembly, the temperature conditioned air arrives at a manifold connecting two Pressure Regulating and Shut-Off Valves (PRSOV). The bleed air flow rate through the system is considerably different in situations when cabin bleed air is only used, or when cabin air together with anti-ice bleed air is used. For this reason two PRSOVs are installed. The Low Flow PRSOV (100 +20/-15 psig) is ON when the bleed air is only being provided for cabin use and when there is min. 7.5 psig of pressure at the valve inlet. When either the Inlet or Inlet and Wing Anti-Ice is in use, the Low Flow PRSOV will be turned OFF and the High Flow PRSOV (115 +12/-20 psig) turned ON. It requires 8.0 psig of pressure at the inlet of HP PRSOV to fully open. Both, high and low flow PRSOVs can never be on at the same time. They can both be OFF if the cabin air and anti-ice systems are both OFF. The High Flow PRSOV is set approximately 15 psig higher than the Low Flow. This provides more air capability during Anti-Icing operations. The electrical logic of the two PRSOVs is opposite. The Low Flow PRSOV is normally open and requires 28V to close. The High Flow is normally closed, and requires 28V to open. The low pressure PRSOV senses the existing 30 psi cabin air regulator to determine the required position. If the Cabin Air switch is moved to OFF, thereby closing the existing 30 psi cabin air regulator, the low flow PRSOV will close. This could result in shutting off all airflow to the cabin from the engine. There are other systems within the aircraft that require a small amount of air at all times (e.g. the door seal). To provide for these systems, a small line (bypass duct on Figure 2), equipped with an orifice, provides flow around the PRSOVs, when both PRSOVs are closed. If the low flow PRSOV was already off because the anti-ice systems were in use and the high flow PRSOV was on, the high flow PRSOV continued to remain on to supply air to the using anti-ice systems. After leaving the PRSOV, the bleed air passes through several safety devices. A mechanical bleed air relief valve is provided to protect the downstream system should the PRSOV fail. Should the pressure exceed 150 ± 15 psig, the relief valve will open and vent overboard. The valve is sized to reduce, but not eliminate the downstream pressure (Figure 2). There is also an overpressure switch installed that will illuminate a light in the cockpit. An over temperature switch is also installed. If this unit senses that the air entering the cabin has exceeded 650° F, it will illuminate a light in the cockpit and shut down the bleed air system on that side to protect the cabin components.


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Before the hot bleed air enters the cabin, the air is moved through the refrigeration unit located in the aft fuselage. The volume of air which enters the refrigeration unit is controlled by the two pressure regulator and shutoff valves (Figure 2). These solenoid-actuated pressure regulator and shutoff valves are installed on the LH and RH bleed air supply lines. These valves regulate engine bleed air pressure to 30 +3/-4 psig (2.1 +0.21 /-0.28 kg/cm2) when DC electrical power is removed. The valves will close and shut off bleed air to the refrigeration unit when DC power is applied. The valves are controlled by the CABIN PRESS switch. The pilot can select either engine or both engines to supply bleed air to the refrigeration unit with the CABIN PRESS switch located on the RH main instrument panel. An emergency pressure valve directs unconditioned bleed air from both engines to the cabin through the emergency pressure duct. Changeover to the emergency system is automatic if the refrigeration unit becomes overheated or overpressurized. If the refrigeration unit malfunctions, an emergency pressurization valve located in the refrigeration unit, allows engine bleed air to flow directly into the cabin. The emergency pressurization valve will not open when the airplane is on the ground. Emergency pressure air is supplied to the cabin through the cabin ceiling outlet. A portion of the cold air is delivered to the cockpit and cabin eyeball outlets through the cold air ducts. Moisture in the cold air is removed by a water separator. A smaller portion of hot bleed air, which is controlled by the temperature control valve, is mixed with the cold air. The operation of the temperature control valve is controlled either automatically or manually by the temperature control system. The conditioned-air is distributed to the cabin floor outlets, cockpit floor outlets, and the windshield and side window defog outlets through the conditioned-air ducts (figure 3).

Figure 2. Bleed air block diagram (Nextant Aerospace Instructions For Continued Airworthiness)


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The air refrigeration unit consists of a cooling turbine, primary heat exchanger, secondary heat exchanger, water separator, aspirator, bypass valves, ejector shutoff valve, thermal switches and a pressure switch (Figure 3). Air flowing through the primary heat exchanger is partially cooled and directed to the compressor where its pressure and temperature are increased. From the compressor, the air enters the secondary heat exchanger where it is partially cooled again. After this second cooling, the air is expanded through the cooling turbine where its pressure and temperature are reduced. Energy from the expanding air is converted to shaft power to drive the compressor. At the cooling turbine outlet, hot engine bleed air is injected through the low limit control valve to mix with the cold turbine air flow to maintain an approximate constant temperature of 39°F (4°C) at the water separator discharge. This air control temperature prevents ice buildup at the cooling turbine outlet and prevents water separator icing. After leaving the cooling turbine, the air flows through the water separator where condensed moisture is removed. Water from the separator is atomized by the aspirator and sprayed into the ram air duct to increase the efficiency of the heat exchangers. The smaller portion of hot engine bleed air is directed through the bypass duct to the temperature control valve, then it is returned to the main airflow downstream from the water separator. Operation of the control valve is regulated by the temperature control subsystem as required to increase or decrease the temperature of conditioned air being delivered to the cabin. The bleed air overpressure switch is installed in a boss in the refrigeration unit. The boss is located at the forward center of the unit, in the plumbing that connects the primary heat exchanger to the compressor inlet. The switch opens when the increasing bleed air pressure at the switch is 53 psi (3.7 kg/cm2) and closes when the decreasing bleed air pressure at the switch is 45 psi (3.2 kg/cm2).

Figure 3. Environmental system schematic (Nextant Aerospace Instructions For Continued

Airworthiness)


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The cabin air pressure is controlled by the cabin pressurization control system. The system's major components are outflow safety valves, cabin air pressure controller, manual control valve, pneumatic relay, altitude pressure regulator, air filters, solenoid valves and a dump valve. The components of the cabin pressurization control system are installed in the cabin pressurization control module, except for the outflow safety valves, cabin air pressure controller, manual control valve and dump valve. The cabin pressurization control module is mounted on the aft side of the forward pressure bulkhead. The outflow safety valves are mounted to the forward pressure bulkhead. The cabin air pressure controller, manual control valve and dump valve are located on the RH instrument panel. This system uses a variable isobaric controller to drive two outflow safety valves through a compensated-type pneumatic relay. Both outflow safety valves modulate air flow discharge from the cabin during normal operation. Either or both valves open automatically, as required, to provide positive or negative pressure relief protection. Both valves are connected to cabin altitude pressure regulators, which automatically override the valve if the control system malfunctions. Ground, takeoff and in-flight operation of the system is further controlled by the ground safety relay, thrust lever limit switch and three solenoid valves. Vacuum is generated by engine bleed air passing through pneumatic tubes into a vacuum jet pump and then into the atmosphere. The vacuum source from the cabin pressurization control system contains a check valve, test valve and vacuum regulator. The vacuum regulator maintains vacuum at 4.75 inches Hg (12.065 mm Hg) less than the cabin pressure. The pneumatic relay controls the outflow safety valve control chamber pressure to the desired level. The relay also controls cabin prepressurization. This control is accomplished by signals from the cabin pressure controller and cabin pressure to the pneumatic relay. The pneumatic relay is comprised of two housings, two diaphragms and a cover. The lower housing contains a metering valve and three connections. The upper housing contains a control connection with an air passage to the cover. A second air passage in the upper housing mates with a lower housing air passage to allow air flow from the lower housing into the upper housing chamber. The upper and lower diaphragms are connected together by a common metering valve shaft. A spring in the cover holds the metering valve in the closed position.

1.7 Meteorological information

Wind at 030⁰, 1,1G1,8 kt; QNH 1042,8 HPa; clouds 9/10 at 1100 ft; air temperature -14,9⁰C.

1.8 Aids to navigation

Not relevant.

1.9 Communications

Not relevant.

1.10 Aerodrome information

Not relevant

1.11 Flight recorders

The aircraft was equipped with both FDR and CVR. The recorders were secured and removed for

readout.

1.12 Wreckage and impact information

Not relevant.

1.13 Medical and pathological information

Not relevant.


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1.14 Fire

Not relevant.

1.15 Survival aspects

Not relevant.

1.16 Tests and research

After the event, the aircraft pneumatic system was tested under Estonian Safety Investigation Agency

oversight. Functional testing and visual inspection of the PRSOVs from both engines was performed,

boroscopic inspection of both precoolers was carried out.

Bleed air pressure and water separator temperature test and the aircraft leakage test did not reveal

any abnormalities in the functioning of the system or any of the subsystems.

The functional tests of the PRSOV`s showed that the LH high flow PRSOV failed once during pneumatic

system checkout.

Precooler boroscopic inspection revealed that multiple cooler springs were separated inside LH engine

precooler assembly (figure 4, figure 5).

Figure 4. Separation of cooler springs in precooler assembly


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Figure 5. Separation of cooler springs in precooler assembly

Both high flow PRSOVs and diverter valves were removed and replaced. The PRSOV’s were sent to

Dukes Aerospace Inc. for testing, while both diverter valves were sent to Williams International for

testing.

Because the high flow PRSOV’s were not flagged by Dukes Aerospace, Inc., as being unit subject to an

investigation warranting close inspection, no incoming test was performed. Therefore, no

determination could be made as to whether there was any failure or malfunction of either high flow

PRSOV.

LH precooler assembly was replaced.

Cabin pressurization leak rate check was performed at FL270, differential pressure 8 psi, with Engine

and wing Anti-ice systems inoperative and cabin air off – thus both, high flow and low flow PRSOVs on

both engines in closed position. The tests showed cabin leakage rate 4500 ft3/min. According to the

STC holders` procedures, the cabin leak rate should not exceed 5000 ft3/min.

Williams International did not reveal any abnormalities in the functioning of the diverter valves.

1.17 Organizational and management information

Not relevant.


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2 Analysis

2.1 General

The flight crew did not report anything abnormal with the functioning of the aircraft prior and after

the depressurization event and anything other than the depressurization of the aircraft.

Evaluation of the meteorological conditions found no relevant abnormalities that could refer to

weather or any other conditions that could have caused or contributed to this incident.

Review of the aircraft technical log and maintenance records did not reveal any relevant issues or

deferred defects. No evidence was found that would indicate that the aircraft had not been maintained

in accordance with applicable regulations.

After the incident, testing and inspecting the aircraft and its systems revealed that the LH HF PRSOV

failed once when performing functional testing and that within the LH precooler assembly there were

multiple loose cooler springs. No evidence was found that the separation of the cooler springs could

have had any effect on the bleed air system.

There was no evidence to suggest that an operational or a human factors related aspect was a causal

or contributory to this serious incident. Therefore in analyzing the causes of this incident, the

investigation focused on technical related aspects.

2.2 Technical aspects

The flight crew of ES-NXT did not detect any anomalies with the aircraft prior and after the

decompression of the aircraft. Rapid decompression of the aircraft happened shortly after the captain

switched off the wing and stabilizer anti-ice systems, while keeping the engine inlet anti icing system

switched on. After the event, during investigation, the cabin pressure leakage with both HF and LF

PRSOVs in closed position was measured (4500 ft3/min) to be similar to the leakage that the flight crew

experienced during the incident (4000 ft3/min). Therefore ESIB decided not to focus in in further

investigations on the cabin pressurization control systems, but on the cabin pressurization and bleed

air systems.

When the aircraft was flying at cruising altitude FL300 with wing, stabiliser and engine de-icing

systems switched on, both engines had the HF PRSOVs open and LF PRSOVs in closed position. Both

inlet-heat PRSOVs were open and the wing anti-ice PRSOV in open position (figure 2). By switching off

wing and stabiliser de-icing system and keeping the engine anti-ice system on, only the wing anti-ice

PRSOV should have closed. Since the engine de-icing system was still kept on, according to the system

logic, to provide sufficient amount of bleed air to pressurize the cabin and provide hot air to the engine

de-icing system, the solenoids in the HP PRSOVs should have been kept energized and the valves open.

However the rapid pressure drop in the cabin refers to the fact that while the wing anti-ice PRSOV was

closed, both HP PRSOVs closed as well, thus cutting off the pressurized air supply completely and

causing the rapid pressure drop in the cabin. Therefore the most probable reason for the

depressurization to occur and both HF PROVs to fail simultaneously at these conditions is that the HF

PRSOVs solenoids must have been de-energized, causing the valves to close.


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3 Conclusion

(a) Findings

1. LH HP PRSOV failed once on functional testing the valve. 2. LH precooler assembly had loose cooler springs. 3. Cabin pressurization leak rate check revealed similar leak rates as the flightcrew experienced

during the flight.

(b) Causal factors

The probable cause of this serious incident is the simultaneous malfunction of both high flow

pressure regulating shut-off valves.

(c) Contributory factors

Debris from the LH precooler assembly might have had affected the effective functioning of the

LH PRSOV-s.

4 Safety Recommendations

It is recommended for Nextant Aerospace to take all necessary actions to ensure that all operators

performing flights with Beechcraft Corporation Beechjet 400A aeroplanes with Nextant

Aerospace L.L.C modifications EASA STC no. 10042353 incorporated into the aircraft are provided

with necessary procedures and checklists for safe operations of the aircraft at cabin pressure loss

as a result of having Wing Anti-Ice and L&R engine Anti-Ice On.

4.1 Actions taken so far

Since this serious incident, the operator has replaced both of the HF PRSOV`s and both precooler

assemblies.

Nextant Aerospace is in process of modifying Cabin Decompression Checklist with the procedures

for loss of cabin pressure as a result of Wing Anti-Ice and L&R engine Anti-Ice On.

investigation report · year of manufacture 2000 total flying time, hours 8,349.73 flying time...

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