Was at EAA AirVenture (Oshkosh) this past week, and at
one of the instrument seminars one of the attendees asked a question I thought
would be interesting to others as well. It was involved, but the part I'm going
to discuss boiled down to the following (paraphrased):
I am based at an airport with some high terrain
nearby that drives up the MDA for the approaches to about 900 AGL. However, I
know that I can depart the airport with 0/0 ceiling and visibility. Why is
that?
The presenter wasn't able to answer the question (I think
due mostly to not understanding what it was), but I was able to help.
First, some background.
As discussed in other blog posts of mine, the MDA (or
other minimums) are often determined by nearby terrain. If you have high
terrain within a few miles of the airport, on final, you will often have high
MDAs for the approach.
This makes sense, and of course 14 CFR 91.175 tells us
that if you need to make an instrument approach, you cannot go below the DA or
MDA unless certain parts of the runway environment are in sight, as every
instrument student learns (I hope).
However, for departure, instrument students are taught
that you can legally take off in 0/0 conditions (zero visibility and ceiling at
zero feet) and what's more, that's true!
91.175(f) states the standard minimums required for
takeoff, such as "aircraft having two engines or less - 1 statute mile
visibility", and also requires that aircraft comply with an obstacle
clearance procedure. HOWEVER, this
subparagraph only applies to "persons operating an aircraft under part
121, 125, 129, or 135". In other words, NOT part 91 operators, which applies
to many of us (including the person asking the question).
Also, from the Instrument Procedures Handbook, page 1-8:
Aircraft operating under 14 CFR Part 91 are not
required to comply with established takeoff minimums. Legally, a zero/zero
departure may be made, but it is never advisable.
So as a part 91 operator, I have to comply with high
minimum altitudes on an approach due to high terrain, but yet I can depart the
same airport with zero visibility? Why is that?
The simplest answer is that the takeoff minimums for part 91 are not determined by obstacles or even evaluated in any
way. 0/0 is the rule only because it's not prohibited by 91.175. You can make
up your own departure minimums if you will.
An examiner once told me to consider three things when
thinking about doing something in an airplane - Is it legal? It is safe? Is it
smart?
Taking off in 0/0 weather may be legal. It might even be
debatably safe-ish, depending on aircraft performance and terrain. It is smart?
I don't think so. Not many "outs" in the event of any kind of problem.
What should you do as a Part 91 operator? In my opinion,
always comply with at least the established takeoff minimums for the runway.
Looking at a specific example, South County Airport in
San Martin, California (E16) has the following departure minimums:
And then has an admittedly lengthy definition of the
departure procedure to follow. But the standard takeoff minimums (the ones in
91.175 that do not necessarily apply to Part 91) can only be used if you can
maintain a climb gradient of 324 feet per nautical mile (not per minute) to a
certain altitude, depending on which runway you depart. Or if you can't meet
that, it allows a departure in visual conditions (called a Visual Climb Over
Airport, VCOA) with at least a ceiling of 1700 and a visibility of 2.5 miles.
This is to get you high enough that once you enter the clouds you can keep
climbing and have a good safe cushion over the nearby terrain. If you can't
meet that, maybe you should wait a while until the weather improves.
But VCOAs are a good topic for another blog. Fly safe!
In your explanation, you state 'this subparagraph only applies to "persons operating an aircraft under part 121, 125, 129, or 135'. However, the word 'only' does not appear in the regulation. Instrument Procedures Handbook notwithstanding, I contend that 91.175(f) DOES apply to Part 91 operations. Other subparagraphs in Part 91 don't say that they apply to Part 91 operations, it's implicit. Why should this subparagraph have to say that it applies, it's published IN Part 91. Legally speaking, and parsing the words, I think that it does apply.....
ReplyDeleteMikeM, thank you for the comment.
ReplyDeleteI see where you're coming from - you're right, 91.175(f) isn't very well worded, and it could be inferred that since it's in Part 91, it applies to Part 91 operations in addition to the other parts listed. However, this isn't the case. In addition to the FAA's own Instrument Procedures Handbook explicity stating that takeoff minimums do not apply to Part 91, I refer you to the following Regional Counsel letter from 1999:
http://www.faa.gov/about/office_org/headquarters_offices/agc/pol_adjudication/agc200/interpretations/data/interps/1999/landis%20-%20(1999)%20legal%20interpretation.pdf
Your concern on wording was identified as early as 1967, and the current wording is apparently the "clarified" version!
Thank you for reading my blog, and please let me know if you have any ideas for topics for future articles. Good flying!
Russ
Thanks for the blog.
ReplyDeleteAt my old airport (KPAO) a fatal accident occurred with a C310 as the pilot attempted a departure in 0/0.
https://eesmog.files.wordpress.com/2011/11/ntsb_e11232011120000.pdf
As I complete my IFR rating now, I always wondered about the legality of 0/0. My CFI thought at least Approach RVR was required but the KPAO tower allowed the 0/0 departure at "pilots risk". My own personal minimum would always be enough visibility to land again if there was a flight issue. Hence minimum approach plate RVR makes sense.
Peter, thanks for reading! The accident report you linked is sobering, and unfortunately we'll never truly know the source of the pilot's spatial disorientation. However, it is well known by IFR pilots that one of the most difficult moments in a flight is that first moment you enter IMC - the transition as you switch from visual to instruments. That's why it's a good idea to get on instruments before you actually enter IMC - so you're already "in the groove".
DeleteWith a 0/0 takeoff, you don't have that option. You have to be visual as you roll down the runway, and as soon as you lift off, you're instantly IMC. Dangerous stuff those next few seconds.
Approach RVR as a minimum at least gives you a little time to get "on the gauges".
Glad you're completing your instrument rating! It's a tough one but a very worthwhile rating to have. Good job!
Recently had a student ask “why is the minimum visibility different depending on the number of engines?” Given that both aircraft are operating under IFR, what is your take on this question?
ReplyDeleteHi Steve! While I don't have any great insight into the "why" of this regulation, I can speculate. In my opinion, the takeoff minimums aren't an issue of "navigation" - the airplane and avionics don't know the weather, and you can navigate the same regardless of the ceilings and vis. Rather, it's from an "emergency" perspective, and primarily an "engine failure" one.
DeleteIn a single-engine aircraft, obviously if the engine fails, there aren't many options, you are going to descend, and having a little bit of visibility is going to help with trying to make a safe landing.
In a multiengine aircraft, of course, in many cases you will be able to keep going, however the concern there is going to be any initial heading changes as you deal with the control forces changing. Asymmetric thrust, etc. Although my experience tops out at light twin-engine jets, I would guess that the more engines an airplane has, the less of a concern this is - for a B-52 with 8 engines, losing one of them probably isn't going to result in much control difficulty.
Since we are visual creatures, having that added visibility requirement for single- and twin-engine aircraft probably helps with being able to control it in the event of an engine failure on takeoff roll or just after takeoff.
That's my guess!
Thanks for reading.
Russ