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3 Answers

What is the point of MOCA and when would you use it as opposed to the MEA?

Asked by: 1709 views Airspace, Instrument Rating

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3 Answers

  1. BJ Miller on Oct 27, 2015

    I would like to go a bit more plain language as opposed to textbook definitions if that is alright. The point of a MOCA is simply as the name implies, to provide a minimum obstacle clearance (2k in mountains, 1k elsewhere) with minimal navaid reception guaranteed. This differs from an MEA in that an MEA is just as concerned with providing line-of-sight navaid identification along the entire applicable segment of the route in addition to MOCA requirements. This is why the MEA is often higher and likely will be significantly higher in mountainous terrain where a terrestrial navaid signal will be blocked by terrain. It’s my understanding that the 22 mile guaranteed reception for a MOCA is based on a rule of thumb for line of sight at that altitude. The actual navaid reception could be much more than 22 miles, but the FAA will not guarantee it and in my opinion should not be used for planning purposes.

    So why care about it and not just use the MEA? Unfortunately, I don’t think there is a textbook answer and in my opinion, it depends on you identify the airway. Even with GPS, I have always used the MOCA as a sort of emergency altitude. If I ever had to get below icing for example and just cared about getting below weather and probably going back to where I came from, I knew an option was to request to descend to the published MOCA and turn around. GPS is very reliable, but if it were to fail while below MEA, I’m likely unable to identify the airway. Another thing to keep in mind, even with the best GPS on the market to overcome the navaid reception problem, ATC may not be able to talk to you at the MOCA (also not necessarily guaranteed at MEA).

    In summation, the point of a MOCA is simply to get you minimum obstacle clearance and minimum terrestrial navaid reception along your route of flight. I personally would only use the MOCA in an emergency situation for the reasons stated above, but others may feel differently. Best advice is to get the opinion of as many instructors as you can.

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  2. Drew on Oct 28, 2015

    Very helpful. Thank you. I thought maybe the MOCA was something left over from the old days of aerial navigation, but it seems it’s still useful for some unique situations like you mentioned.

    I believe there is some regulation saying that you must use have the primary mode of navigation if you are tracking an airway (such as the VOR); is that correct? So you if you were using the GPS to track an airway, you are also required to have the VOR operating/available? If you were only using the GPS, you would not be able to fly an airway?

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  3. BJ Miller on Oct 28, 2015

    Is this the regulation of which you speak? If not, please clarify.

    14 CFR Part 91, §91.177 Minimum altitudes for IFR operations
    (1) The applicable minimum altitudes prescribed in parts 95 and 97 of this chapter. However, if both a MEA and a MOCA are prescribed for a particular route or route segment, a person may operate an aircraft below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR (based on the reasonable estimate by the pilot operating the aircraft of that distance)

    If your GPS is not certified for IFR flight, then you cannot use it as a primary means of airway navigation. This means that you are required to have an operable VOR and receiving reliable signals to navigate the airway. The above regulation places limits on what that means at the MOCA.

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