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4 Answers

Minimum Flight Level adjustment

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Instrument Rating

The following is stated in the air traffic control procedures; what does it mean to the pilots?


When the prescribed minimum altitude for IFR operations is at or above 18,000 feet MSL and the atmospheric pressure is less than 29.92”, add the appropriate adjustment factor from table 4−5−3 to the flight level equivalent of the minimum altitude in feet to determine the adjusted minimum flight level.

Altimeter Setting        Adjustment Factor

29.92” or higher          0000 feet

29.91” to 29.42”         0500 feet

29.41” to 28.92”         1,000 feet

28.91” to 28.42”         1,500 feet

28.41” to 27.92”         2,000 feet

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4 Answers

  1. Best Answer

    John D. Collins on Jul 08, 2013

    Flying in the flight levels means you are flying with the altimeter set to a fixed value of 29.92. Flying using MSL requires that the altimeter setting be adjusted to a nearby and current altimeter setting. Below FL180, the next lower VFR altitude permitted is 17,500 MSL. There are no altimeter setting limits on the use of MSL altitudes as they are adjusted to the local altimeter setting. Because flight levels use a fixed altimeter setting, when the local altimeter setting is lower than 29.92, the vertical separation between aircraft flying the lowest flight level and VFR aircraft flying the highest MSL altitude is reduced. The reduction in vertical separation is approximately the difference between 29.92 and the current altimeter setting times 1000. So at an altimeter setting of 29.42, a VFR aircraft would be at the same altitude as an IFR aircraft at FL180 ((29.92-29.42)x1000 = 500 feet). To regain the minimum vertical separation, FL180 may not be used and IFR aircraft would be assigned FL190.

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  2. Lion on Jul 08, 2013

    Thank you John. You made it clear.

    So, this is an ATC function and pilots don’t have to do anything about this altitude adjustment.

    But this makes me wonder; what if the VFR aircraft is:

    – Setting 29.42
    – Cruising at 17500
    – Not not being detected by the ATC

    While the IFR aircraft is:
    – Setting 29.92
    – Cruising at its assigned FL 180

    Does this mean there is a possibility of traffic conflict between these two aircraft!?

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  3. John D. Collins on Jul 08, 2013

    The main action the pilot needs to take is one of planning. If they had planned on using FL180 but the lowest FL available for the direction of flight is FL200, they have to adjust their planning (climb time, performance, winds, flight plan, etc.).

    ATC will not clear the aircraft to use FL180 under these conditions or if they occur enroute, will require the aircraft to change to a lower MSL or highe FL altitude.

    If the conditions were as stated, there would be no vertical separation between a VFR aircraft at 17500 feet MSL and an IFR aircraft at FL180. If their paths crossed, they could collide, since they are at the same actual altitude.

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  4. tommytom on Jul 12, 2014

    When applying the minimum adjustment is the altimeter based on what it was prior to take off? Also, If I was flying a long cross country across the US at level cruise FL180. When I departed the airport the altimeter setting was above 29.92 not requiring a minimum flight level adjustment. 3 hrs into the trip the altimeter setting in the area is below 28.41. Would ATC be aware of this and request that I climb maintain FL 200?

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