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Mode Aviation
Reference No. AR201400030
Date reported 19 June 2014
Concern title Changes to operator’s maintenance checks
Concern summary

The concern related to the changes the operator has made to the Boeing 737-800 aircraft maintenance procedures and how this affects the ability to ensure that there is sufficient oil in the engine to operate safely.

Industry / Operation affected Aviation: Air transport
Concern subject type Aviation: Maintenance

Reporter's concern

The reporter expressed a safety concern that the new 'Check 2' requirement, specifying that the engine oil levels are required to be checked every 48 hours, does not ensure that the minimum oil level requirements from the manufacturer are being met.

The reporter advised that with the increased flying requirements for the Boeing 737 fleet and the new 'Check 2' requirements, there is an increased risk that the aircraft are operating with oil levels below the minimum levels required. The Boeing Maintenance Manual for the B737-800 recommends that the engine oil tank servicing be completed daily after the last engine shutdown of the day or that the frequency can be increased depending on the operator’s aircraft usage (AMM CH.12-13-11-600-801 PARA.2. A.(4)&(5)).  The reporter advised that as the aircraft are being flown to capacity, the requirements of the manufacturer are not being fulfilled.

The reporter also advised that as the oil levels are not being checked daily, the flight crew are now the last line of defence, but the minimum oil requirements are not specified in the flight crew operations manual (FCOM), neither is the B737-800 engine oil usage rate and how to calculate oil consumption on a flight leg. The Boeing Aircraft Maintenance Manual (AMM) Task 12-13-11-600-801 part 2.G (1), Subtask 12-13-11-970-004, requires that this is checked prior to dispatch and calculated for the flights duration. This information is currently not specified in the FCOM to allow engine oil usage calculation and minimum requirement.

The Maintenance Instruction, for Engine Oil Level and Consumption Guidelines, implies that oil level checks must be conducted within 5 to 30 minutes after engine shutdown at the end of the day, which contradicts the new 'Check 2'.

The reporter also advised that to comply with extended diversions time operations (EDTO) requirements, the LAME must enter the engine oil uplift into the online technical log daily, to allow the correct functioning of the company Engine Condition Monitoring System.

Other operators within Australia of the Boeing 737-800, who operate on a 48 hour check system, ensure that the engine oil is checked daily at their base ports. This ensures that aircraft are EDTO compliant for the following days flying.

Operator's response (Operator 1)

The new 'Check 2' requirements (from 36 hrs. to 48 hrs.) are based on the Boeing B737-800 Maintenance Program Document (MPD). Presently the Aircraft Maintenance Manual (AMM) states ‘Oil tank servicing is usually done daily, after the last engine shutdown of the day’. The AMM makes allowance for more frequent checks 'depending on the operator's airplane use rate'.

Oil uplift is reported via the on-line tech log. Our average oil consumption on the B737 fleet is 0.25 L/Hr.  Based on a tank capacity of 20.4 litres and 7.6 hours per day per aircraft utilisation approximately 3.8 litres will be consumed within 48 hours (16.4 litres remaining). The AMM requires a minimum of 6.65 litres available at the end of the flight and consequently our aircraft would be well above the specified limit. Engines with a higher than desirable oil consumption trend are identified and subject to additional servicing.

In regard to flight crew, they are not the 'last line of defence' in this case. The approved System of Maintenance includes monitoring functions and capabilities as detailed previously.

Flight crew do not, and should not, be concerned about the engine oil consumption rates. This is the function of the engineering. We have provided flight crew with a minimum dispatch value supplied by the engineering area, which is quickly and easily read from the Display Unit. This is also the case for flight crew oxygen, and has been the case for some time. While flight crew oxygen levels are read from the overhead panel, the philosophy of providing a minimum dispatch value for an important aircraft system is not unusual, nor is it without precedent.

The time interval related to oil quantity checks after shutdown pertain to avoidance of over-servicing (filling). The pre start value stated is conservative. If the aircraft is dispatched at the minimum value, it will comply with all other AMM requirements (pre-take-off/acceleration and final approach/missed approach at destination). If the subject aircraft displays a value less than the minimum prior to engine start from a non-maintenance port, the aircraft will be unable to depart. Although it may have commercial implications this is not a safety of flight issue.

As part of the Post Implementation Review (PIR) process, the subject of dispatch oil quantity is being raised at all flight crew simulator cyclic and route checks to ensure a thorough understanding. Latest feedback from Flight Training is that all flight crew are monitoring the oil quantity, understand the policy and have not raised specific any concerns now that background and policy information has been published.

Finally, flight crew have been advised that if any doubt exists, they must initiate contact with Engineering and ask all necessary questions.

In summary, Oil quantities are checked at following intervals:

  1.  Pre-flight check by pilot as per FCOM procedure occurring prior to each departure to ensure sufficient oil quantity
  2. Check 1 prior to EDTO flight
  3. Check 2 every 48 hours

Regulator's response (Regulator 1)

CASA is satisfied that the revision of the maximum elapsed time between 'Check 2' maintenance inspections from 30 to 48 hours for the Boeing B738 aircraft meets the manufacturer's recommended requirements. The operator has appropriate systems and processes in place to manage the revised 'Check 2' oil level check schedule.

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Last update 15 May 2015