Thursday, July 28, 2005

Contradictory Information

Comments on my last entry suggest that I have misinterpreted the IFR rules of my nearest neighbour. I don't feel too badly about this, as I know my own country's rules well, don't hold a US instrument rating, and don't currently operate IFR in US airspace, but I'm getting some contradictory information from some experienced US pilots, and I want to straighten it out.

Old Blind Dog, who has been flying aircraft of all sort for a lot longer than me, contradicts what I wrote in my last post, saying "the restriction is applicable to part 135 or 121 operations but not to part 91 corporate or private operations" and "RVR is only controlling prior to commencement of approach" and "For operations under part 91 I can commence the approach no matter what the reported weather."

I'm looking at Part 91 of the Federal Aviation Regulations. (As I admitted earlier my copy of the FAR/AIM is from 1998, so I welcome corrections of fact as well as interpretation.) According to 91.101, Section B applies to "aircraft within the United States and within 12 nautical miles of the coast." No mention of specific commercial operations.

91.175 (c) states that "no pilot may operate an aircraft, except a military aircraft of the United States, at any airport below the authorized MDA or continue an approach below the authorized DH unless --"

Three necessary conditions follow: stability of the approach, visibility, and visual reference. The visibility condition reads, "(2) the flight visibility is not less than the visibility prescribed in the standard instrument approach being used."

The next bit is "91.175 (d) Landing No pilot operating an aircraft, except a military aircraft of the United States, may land that aircraft when the flight visibility is less than the visibility prescribed in the standard instrument approach being used."

91.175 (f) Civil airport takeoff minimums appears to confirm David's comment that part 91 aircraft have no legal take off limitations at all.

I'm glad that what the experienced guy is telling me corresponds more closely to my idea of safety than my interpretation of the FARs, but I want to know how to get what he is saying out of what I'm reading. And yes, I know the joke about what happens if pilots trying to understand regulations, but I'm willing to risk it.


Avimentor said...

Here is a link to the current U.S. regs:

As to clearing your confusion, here's my attempt. The relevant regs are 91.175, 135.225, 121.651.

Part 135 operations are governed by a combination of part 91, part 135 and the certificate holder's FAA-approved operations specification. Generally, part 91 is the foundation with part 135 and the operations specification providing further restrictions. Sometimes the operations specification relieves some of the restrictions.

Under part 135, a pilot may begin an approach if the latest surface weather report indicates the conditions are at or above "authorized IFR LANDING minimums for that airport." (emphasis mine) So if you listen to the ATIS, AWOS, ASOS and the reported VISIBILITY or RVR is below what is specified on the approach plate, you can't begin the approach under 135.

Under part 135, if a weather report of worse conditions is received after begining the final approach segment (there a a bunch of stipulations), the pilot may continue the approach and, upon reaching the DH or MDA, may land if 91.175 LANDING minimums are met.

91.175(d) landing requirements are based on FLIGHT VISIBILTY, not ceiling. This is true for 135 commercial operations and non-commercial GA pilots. For the purposes of approaches, 91.175 talks about operating below the DH or MDA and then it talks about landing requirements.

Many certificate holder's approved operations specifications allow lower visibility takeoffs than standard or published minimums. I don't know of any 135 operators who are allowed to conduct 0/0 takeoffs, though.

Non-commercial operations under part 91 can takeoff in 0/0 and attempt an approach if the required landing visibility is not being reported, but I'm told there are cases where a pilot came to grief after a 0/0 takeoff and the FAA used the 91.13 "careless and wreckless operation" in an enforcement against the pilot.

Hamish said...

Here's my take on this: Part 91 is the broadest set of requirements. As an IFR pilot flying under Part 91, I can commence and fly an approach all the way to MDA or DH regardless of the reported visibility or ceilings; at decision time it's the actual flight visibility and ceilings that matter.

Under the far more restrictive Parts 121 and 135, which build on the basics in Part 91, I can't even commence an approach if the reported visibility or ceilings are lower than the approach's published minimums. Once I've commenced the approach, I can't continue it if reported minimums are lower than published unless I'm past the FAF. At which point (simplifying a bit), actual flight visibility and ceilings become the ruling factors.

But I'm probably wrong, and John will save the day with a more authoritative description...

(Argh -- looks like he just did :-) ).

Old Blind Dog said...

The key word here is flight visibility, i.e. what the pilot sees, not what some guy (or machine) on the ground sees.

Aviatrix said...

I completely understand that part 121 and 135 requirements are in addition or exception to the part 91. We have exactly the same thing: all the way through the commercial operations rules are the words "unless otherwise specified in the air operator certificate." The part I still don't understand is that you all maintain that a pilot operating under part 91 can continue an approach and land regardless of the visibility, despite 91.175.

If the visibility referred to is flight visibility determined entirely by the pilot, why does the rule exist, especially (d)? Is a pilot who has the required visual reference to complete the landing, supposed to verify his flight visibility before touchdown? When the visibility is low, it's pretty hard to say what it is. I'm flying in one mile vis, coming in over the lake with nothing to see within one mile ahead. I have no idea whether I have one mile, half a mile or 500 feet. It's just grey.

Specifically, why do 91.175(c)(2) and 91.175 (d) seem so restrictive if the intent is to let the private pilot choose his own weather?

Avimentor said...

Even a pilot governed under part 91 cannot operate below the DH or MDA nor can s/he land unless they meet the requirements of 91.175. The only differences for part 91 ops are being able to commence the approach, that there need not be any surface weather reporting at the airport, and that a 0/0 takeoff is not strictly forbidden in the regs.

Old Blind Dog said...

Your question is "how do they know if the pilot is cheating and descends below DH or MDA without the required flight visibility to continue the approach?" and the answer is that they don't, unless he/she puts it into the ground off airport. The majority of pilots don't cheat because they know it is dangerous but there are always some that do. If you find yourself working with them...get a transfer asap.

Sam Weigel said...

Hamish, under Part 121 and 135 we cannot begin the approach if reported vis or RVR is below minimums, but reported ceiling is not controlling.

Aviatrix, I think the main difference between the US & Canada is reliance on the reported weather. In the US, Pt 121/135 operators need reported vis to start the approach, and that's how the regs keep us from "cheating." If the vis comes down inside the FAF, the situation is obviously fluid, and the regs give us the leeway to take a "look-see" just like any Pt 91 pilot could.

As far as how we're supposed to determine flight vis in an instant when we break out at mins...well, if we're at 200 ft on the ILS and we see the runway, it's safe to say we have our half mile. For other distances, the standardized approach lighting systems and runway markings are great for estimating distance.

In my opinion, the reason that US regs are more lenient is the greater use of automated weather systems. We have a ton more instrument approaches than Canada, and only a portion of them are at airports staffed by human observers. AWOS/ASOS is pretty weak when weather conditions are close to mins, and many of them don't get all the maintenance they should. Thus, greater flexibility to "look and see" for FAR 91 operators, and a greater reliance on "flight visibility" than "reported visibility" for all of us.

Old Blind Dog said...

Except that we have always had the ability to execute an approach procedure even before the machines came along when operating under part 91. Before the AWOS/ASOS machines part 121/135 operators were unable to use many airports, except in VMC conditions, because there was no weather reporting available and they were, therefore, unable to commence the approach.