Serious flaws in Iran Air's fleet maintenance and airworthiness programmes led to the decision to ban most of the flag-carrier's aircraft from European Union operations.
Iran's Civil Aviation Organisation has demonstrated "several weaknesses" in its oversight of Iran Air, including a lack of detailed review of maintenance programmes and minimum equipment lists.
These failings have led to "failure to detect errors" made by the airline, states the European Commission in an assessment detailing its rationale for blacklisting the carrier.
Iran Air is suffering "significant deficiencies" in the management of airworthiness and maintenance, it says.
"In particular basic errors had been made in the maintenance programmes leading to significant omissions from the programmes for safety-related equipment on the Airbus A320 fleet and the Boeing 747-200 freighter," it adds.
Ramp check results from Iran Air's A320s are "noticeably worse" than those for other aircraft in the airline's fleet.
Assessment of the airline's operations has shown that the A320 fleet maintenance system "failed to ensure that deferred items had been rectified in the specified timescales".
No flight-data monitoring has been carried out on the A320 fleet while the rate of data-gathering on other aircraft has been "very low", the Commission adds: "Moreoverthe company is failing to address the basics in terms of the continued airworthiness of its aircraft."
Iran Air's A320s, along with its Boeing 747s and 727s, have been included in the most recent revision of the European Union blacklist.
The Commission acknowledges that, during an assessment visit in May-June, the Civil Aviation Organisation was able to demonstrate an oversight system which "complies with the intent" of ICAO regulations, and that the CAO had a "strong commitment" to adopting modern safety management techniques.
It adds that the CAO has corrected previously-identified weaknesses in audit follow-up procedures, through a system which enables urgent safety issues to be addressed quickly, and says the CAO has shown an "open, co-operative and constructive approach" to addressing shortfalls.