Dave S.
Pre-takeoff checklist
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- Feb 28, 2017
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thetexan
That is the altitude to which you must* maintain the increased rate of climb in the event that the weather minimums are less than 300-2 but the weather meets the standard minimum for your aircraft. Once you have achieved that altitude, you may decrease your rate of climb to 200 ft/nm. If you can't maintain that rate of climb to that altitude, you can't depart IFR. Aircraft climb performance generally degrades with altitude, so it isn't out of the question that a particular aircraft can make that necessary climb gradient down low but can't at altitude. It makes sense that the FAA let's the pilot know the altitude at which they can safely reduce their climb so they can calculate based on conditions that whether they can make the necessary climb to that required altitude. But that 1300 number has nothing to do with making your turn.
The altitude that you may turn depends on your direction of turn. If to the right, its 1400. Otherwise, its the standard of 400 feet. You could get the clearance to depart 17, and turn left to heading 150. You would climb to 400, turn to 150, but you would want to keep your rate of climb up until you pass 1300, even though you made that turn at 400, so that you clear that power line and the towers to the south and to the east depicted on the VFR chart.
(*Of course, part 91 is not required to follow the alternative take-off minimums. So, from a legal requirement, we are only talking 135/121, etc. That being said, obstacles don't care if you are part 91, so it probably makes sense to observe these minimums, required or not. )
This the point of the intial post.....
Take two examples.
1. A standard cg of 200 fpnm to 400 feet, and
2. A non-standard cg of 512 to 1300 feet
AIM..."...
climbing to 400 feet above the departure end of runway elevation before making the initial turn. ..."
The AIM clearly explains the 400 as a the minimum altitude (as far as standard minimums are concerned) before a turn on course should be started; in this case a diverse departure. If 400 means that in a standard minimum then logic follows that 1300 means the same thing in this non-standard minimum. Yes, it all serves as a minimum requirement for IFR departure as I stated in the original post. In other words, before I can start the takeoff roll I must have 1 mile viz (Im in a Baron) and my Baron must be capable of 200fpnm up to an altitude of 400 feet. Or, if I use the non-standard minimum here, my baron must have the performance capability to climb at 512fpnm all the way up to 1300 feet before I can start my roll. My Baron might be able to do that but my 172 may not in which case I would have to consider the other alternatives. This is a minimum requirement for departure BUT it is also a control requirement which states 'now that you have departed you must actually fly your airplane at the numbers used in the minimum, 512 and 1300'.
Clearly the 1300 is a turn altitude or an altitude, once reached, which allows for a turn. AND, unless otherwise indicated, that turn can be a diverse turn in any direction.
Now let's add the DP which says to fly heading 165 to 1400 BEFORE TURNING RIGHT. Let's to to the AIM once again...
"...A greater climb gradient may be specified in the DP to clear obstacles [the 1400 in our example] or to achieve an ATC crossing restriction. If an initial turn higher than 400 feet above the departure end of runway elevation is specified in the DP [and it is...1400], the turn should be commenced at the higher altitude. ..."
This explains the need to use 1400 instead of 1300 for our altitude to commence a turn. The rub is that it applies only to right turns!!! Putting it all together how do we interpret the entire departure process? Can we turn at 1300 unless it's a right turn then we must go to 1400?
Here's a better question. If 1300 represents the altitude to which we must be able to climb at 512fpnm and thereafter 200fpnm (as you assume)...then at what rate of cg do we climb after 1300 up to 1400? 512? 200? And if 200 what of the obstacle? Has it been cleared sufficiently that climbing at only 200 will suffice as we go from 1300 to 1400. Remember, Im still runway heading at 1300 and have presumably cleared the obstacle. If I cant turn (if Im going right) until 1400 then I continue runway heading until 1400. But wait!!! I cleared SOME controlling obstacle at 1300 and would be able to reduce to 200 fpnm if I continued straight out. So how fast I go from 1300 to 1400 should be irrelevant.
I believe the answer lies in the purpose of the takeoff minimum and the DP. The takeoff minimum gives us a brake release minimum AND includes a couple of control instructions. Those being 1) how fast to climb your airplane and 2) when you may turn your airplane (control instructions are not normally thought of as a "minimum"). The DP gives us nothing but control instructions and uses the cg rates specified in the TO minimum to do so. When TO and DP exist together the DP supersedes the TO minimum to the degree it intends. For example, you could turn on course in any direction at 1300 were it not for the DP which requires that if you intend to turn right then wait until 1400. Since all of that couldn't be wrapped up in a tidy TO minimum description then a DP must be used to clarify.
I'm picking on 50F only because it's a simple example. The reason this needs to be clearly understood is that there are hundreds of other examples where differences between non-standard minimums and DP requirements are equally or more confusing and with greater differences. So let's not get hung up on Bourland for the sake of Bourland.
And let me remind you again, a FSDO 135 POI and the gentlemen that builds these very minimums didn't agree and the interpretation. Although I gave more credence to the person in operations group his answers were sometimes iffy and didn't address any of the other issues I have brought up. It was like I stumped him on some of them.
And we used Bourland as the example.
tex
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