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Probable causes:
- Controlled flight into terrain during the execution of the IFR departure procedure VVC2A, during which the crew mistakenly turned left, contrary to the procedure, heading towards the mountainous area at the foothills of the eastern range, where the impact occurred.
- Loss of situational awareness by the crew, which, for reasons that could not be determined, apparently made a controlled left turn, contrary to the indications of the VVC2A departure procedure, even though it was an experienced crew familiar with the operating area.

The following contributing factors were identified:
- Lack of operator standards, as there was no detailed, organized, and sequential instructional plan and syllabus for the crew to follow during each maneuver, such as the VVC2A instrument departure.
- Lack of operator standards, as there was no specific syllabus for the planning and execution of the Recurrent Check, taking into account, among other aspects, the composition of the aircraft crew, consisting of two instructor pilots, one of whom was conducting the check on the other.
- Inadequate planning and supervision of the training flight by the operator, as they did not conduct a specific risk analysis of the flight, did not monitor its preparation and execution, did not provide details in a flight order or other document, considering especially the composition of the aircraft crew, consisting of two instructor pilots, one of whom was conducting the check on the other.
- Deficient planning and preparation of the flight by the crew, as they informally changed the VFR Flight Plan to IFR, apparently did not conduct a complete and adequate briefing, were unaware of or did not consider the VVC2A SID for the start of the IFR flight, and omitted several IFR flight procedures.
- Crew's neglect of the following IFR flight procedures:
- Not specifying a route and an IFR departure procedure in the IFR Flight Plan.
- Not requesting complete authorization from ATC to initiate an IFR flight. At no time did they mention the VVC2A departure, which was key to the verbally proposed plan before takeoff.
- Not defining or requesting from ATS which standard departure procedure or other they would use to initiate the IFR flight, in which they would encounter IMC shortly after takeoff.
- Not requiring ATC to assign a transponder code before takeoff or at any other phase of the flight, or selecting code 2000 as they did not receive instructions from ATS to activate the transponder.
- Likely not activating the transponder before takeoff and/or not verifying its correct operation before takeoff or immediately once the aircraft was in the air.
- Inaccurate use of phraseology with non-standard terminology in their transmissions with ATC.
- Insufficient experience and training in IFR flights by the crew, despite their extensive experience with the equipment. Much of this experience had been gained in the eastern region of the country, where the majority of DC3 flights are conducted in VMC and under VFR, with no opportunity for the practical execution of IFR procedures.
- Overconfidence of the crew, influenced by factors such as the high flight experience and DC3 equipment experience of the two pilots in the crew, their status as instructor pilots, the relatively low operational demand of the flight mission, and the knowledge, familiarity, and confidence of both crew members with the aerodrome's characteristics, the surrounding area, and especially the peculiarities and risks of the terrain to the west of the takeoff path.
- Non-observance by air traffic control of the following IFR flight procedures initiated by HK2820:
- Failure to issue complete authorization to the aircraft for the IFR flight before initiating the flight or at any other time.
- Failure to issue a standardized instrument departure, SID, or any other safe departure procedure to the aircraft. At no time did ATC mention the VVC2A departure, which was crucial for carrying out the plan verbally proposed by the crew.
- Failure to provide the aircraft with a transponder code before takeoff or at another appropriate time, or to verify its response. This process started only 03:11 minutes after the aircraft took off, so positive radar contact verification was only achieved 04:58 minutes after takeoff, delaying radar presentation and limiting positive flight control.
- Late transfer of aircraft control from the Control Tower to Approach Control (03:35 minutes after takeoff), not immediately after the aircraft was airborne as it should have been, considering prevailing IMC flight conditions in the vicinity of the aerodrome.
- Operating with an incomplete radar display configuration in Approach Control, with insufficient symbology, depriving control of references and judgment elements for an accurate location of the aircraft and its left turn from the path.
- Failure to observe radar surveillance techniques and procedures.
- Inaccurate use of phraseology with non-standard terminology in their transmissions with the aircraft.
- Lack of situational awareness by both the crew and ATC during a flight that, perhaps because it seemed routine, led both parties to omit elementary IFR flight procedures, disregarding the inherent risks of an operation in IMC conditions, with strict IFR procedures that needed to be followed, considering, among other things, the aerodrome's proximity to a mountainous area.

The investigation determined that the accident was caused by the following probable causes:
- Loss of control in-flight as a consequence of the decrease in minimum in-flight control speed and drag generated by the impossibility of performing the No. 1 engine propeller feathering in the face of engine failure.
- Malfunction of the lubrication system of engine No. 1, evident in the abundant oil leakage from the engine, in flight, and in the governor of the left propeller; although discrepancies were found in the maintenance of the propeller feathering pressure line, it was not possible to determine the origin of the oil leakage.
- Weaknesses in the aircraft Operator's operational procedures, lacking a standard that would facilitate a crew's decision making to act in the event of critical failures, in matters such as making an emergency landing on unprepared field or the selection of an alternate airfield.

Contributing Factors:
- Deficiencies in standard maintenance practices during repairs performed on the No. 1 engine's propeller propeller feathering oil pressure line.
- Non-compliance with an effective and reliable maintenance program, which did not verify the operating conditions of the aircraft components; it was not possible to determine compliance with the last 50-hour service, Phase A, to engine No. 1 according to the company's maintenance program, since there are no records of that service in the Flight Log.
- Inefficient safety management system of the Operator for not detecting errors in the maintenance processes and in the conduct and control of operations.

The following factors were identified:
- The lack of technical knowledge published in the POH for the execution of the pertinent actions during the failure of the engine in flight, together with the unwise decisions made by the crew in that situation.
- The haste of the crew members to land caused them to act in an uncoordinated manner and without the assertiveness required for the execution of the procedures contemplated by the manufacturer, the navigation charts and the published approach procedures.
- The omissions, reactions and deviations inappropriate to conduct the flight safely to the runway.
- The turning to the runway on the same side of the inoperative (critical) engine and maximum power on the operational engine during the unstabilized approach to the runway threshold which led to loss of control of the aircraft in low altitude flight.

Controlled flight into terrain after the crew decided to continue the flight under VFR mode in IMC conditions. The following contributing factors were identified:
- Erroneous navigation,
- Low situational awareness regarding the geographical orientation at the beginning and during the flight,
- Failure to observe the warnings of closeness with terrain for three minutes, issued by the TAWS equipment.

The decision and inappropriate operation of the pilot to return with a right hand turn with an uncontrolled fire in the n°2 engine leading to a loss of lift on the wing on that side and a loss of control over the aircraft. It was also determined that the aircraft was not airworthy at the time of the accident. The crew flying experience could not be determined as the crew failed to complete their personal logbook. Also, the captain's medical certificate was not valid anymore.

The following findings were identified:
- Wrong approach configuration on part of the crew,
- Poor approach planning and execution, causing the aircraft to be unstable on short final,
- The landing speed was excessive and the aircraft landed too far down the runway,
- Unnecessary shut down of the engine n°2, causing the hydraulic systems of the main brakes to be unserviceable,
- Late application of the brakes after touchdown.

It was determined that the vibrations and the subsequent failure of the left engine was the consequence of a bad adjustment of the intake valves on cylinders n°8 and 9, which remained stuck in open position, causing high temperature and a loss of power.