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6 Answers

IFR Departure Mins.

Asked by: 668 views FAA Regulations

Why are the standard takeoff visibility mins. different between aircraft with 1 and 2 engines (1 mile) vs. 3 or more engines (1/2 mile)? I am curious as to the FAA logic. Perhaps something to do with climb performance?


Thanks for the feedback.

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6 Answers

  1. John D Collins on Feb 13, 2017

    Dan, you asked this exact question before and got zero responses with 280+ views. Apply a little common sense and either answer it yourself or write the FAA.

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  2. Dan Chitty on Feb 13, 2017

    I guess I have no common sense.

    This will be the final post for me.

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  3. Lemontree on Feb 15, 2017

    I’m not an instructor. I’m just writing to develop your question a little bit further. My best guess on this subject is the same as Dan’s: the difference in climbing performance. Since you can climb faster with more engines, you’re less likely to have obstacles in your climb path, and probably you’ll be able to avoid them more quickly once you have them in sight. But if you think more about it, aircraft with three or four engines are typically heavier than those with one or two, and thus the overall difference in climb performance between the two types of aircraft cannot be so great, and sometimes 3-4 engine aircraft may even climb slower than 1-2 engine aircraft depending on their takeoff weights.

    John says use of a little bit of common sense will give us the answer, but I’ll have to agree with Dan. This question is not as easy to answer as instructors might believe. I hope John or somebody could clarify this for us. Thank you.

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  4. John D Collins on Feb 15, 2017

    I have never seen anything written on this topic that is generally available. If I wanted to get a good answer, I would research it by contacting the FAA. I answer questions that an instructor is expected to be part of their knowledge along with others that go beyond this expected knowledge base if I have personal expertise, such as with TERPS and technical topics on avionics.

    The reason the question was unanswered when it was asked a short time ago is an indication that it is outside of the normal and even expanded knowledge of most instructors. Look thru the unanswered questions and you will see similar themes when questions go unanswered. Sometimes questioners who ask tons of detail questions, many of which can be answered in the regulations or in the AIM, are not willing to do their own research and I would classify them as intellectually lazy.

    If I were to guess, increased visibility enables the pilot to see and avoid obstacles, particularly low and close in to the departure end of the runway. So if you loose an engine with each of the following aircraft, what is the effect on your climb gradient and climb power?

    Single engine – goes from positive to very negative, no power available.

    Twin engine (piston) – goes from 1500 FPM to 200 to 300 FPM at sea level and in some cases climb is not possible. This is because about 35 to 40 % power is required for level flight, leaving 60% available for climb at sea level. Losing one engine can reduce available climb power to 10% or 1/6th of the climb gradient. Add in the extra drag or altitude or temperature, and climb rate rapidly dissipates.

    Twin engine (turbojet or turbo prop) – climb gradient reduced substantially, assuming 35% power is required for level flight, 65% is available for climb with two engines. With only one engine operating, only 15% is available for climb. This suggests that climb gradient is 1/4th of all engines operating

    Three engine turbojet – level may be sustained with one engine and the other two are all for climb. Reduce the number of operating engines to 2, and only half of the climb gradient is available.

    That is my common sense analysis.

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  5. Lemontree on Feb 15, 2017

    Thank you John. I totally misunderstood your original answer. I thought this question went unanswered because it’s too easy to answer. Never imgained it was beyond the normal scope of instructors. But I find your guess quite convincing. Thank you.

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  6. 172Pilot on Feb 18, 2017

    My common sense answer is that it has to do with both a plane’s ability to maintain the TERPS requirements for ROC and separation from the OCS or to maneuver around obstacles that penetrate the OCS in the event of engine failure or power loss. That said, for us Part 91 pilots (who have no restrictions) the usual recommendation is to consider the height of obstacles around your airport of departure and set your personal visibility requirements based on familiarity with the area and availability of safe off-airport landing sites. My experience is that even with a 2000′ ceiling, 1 mile visibility still looks pretty gray at ground level – but that’s me.

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