Visibility to land on Pt 121?

[mmmdonut]

Good math, but you measured from the wrong point.


TDZ to start of runway : 1000'

Rope extended from end of aimpoint markings to point in space along glide slope at DA (200') : way more than 1000'

I think you measured to TCH. (approx 50')

keep workin on it though.

 

[Andy Neill]

So if you saw the fixed distance markers (1000' down the runway) at a DH of 200 above touchdown, then the run to the asphalt would be 200/sin(3 degrees) = 3821' (3827' flight vis). If you had descended to 100 above touchdown before seeing the fixed distance markers then the run to the asphalt would be 1911' (1913' flight vis). I don't see what either one of those calculations has to do with the point at hand, however.

If I were wearing the 4 stripes, and I saw the required amount of visibility from my nose to the ashphalt, I would land regardless of what the RVR or prevailing visilbility was being reported. If I couldn't see that far down the runway, I wouldn't even if the RVR was reporting above mins.

 

[A Squared]

Good math, but you measured from the wrong point.


TDZ to start of runway : 1000'

Rope extended from end of aimpoint markings to point in space along glide slope at DA (200') : way more than 1000'

I think you measured to TCH. (approx 50')

keep workin on it though.

Ahhh, OK, I misunderstood you the first time around, I thought that you were trying to say that the flight visibility was more than the ground visibility because it was measured along the glidepath (sloping) rather than horiozontally.

 

[mmmdonut]

Ahhh, OK, I misunderstood you the first time around, I thought that you were trying to say that the flight visibility was more than the ground visibility because it was measured along the glidepath (sloping) rather than horiozontally.

Naw... skeezer was right.. this is just a commonly miscommunicated subject. I'm confused!!

 

[atldc9]

On another note, for Cat II or III ops, if you receive a report of required RVR being less then required for the approach at any point prior to touchdown, you must execute a missed approach. The old rule of once inside the FAF does not apply to CAT II and III ops.

 

[FlyChicaga]

This is all pretty simple, and it seems people are making it more difficult with Trigonometry. Did I even spell that right?

If you are outside the FAF on an instrument approach, and the visibility reported or RVR reported is below minimums, you must discontinue the approach.

If you are inside the FAF on an instrument approach, and the visibility reported is below minimums, then you can continue to DH. If, at DH, you have the approach lights in sight, you can continue down to 100 ft above the touchdown zone. At 100 ft, if you do not have the runway environment in sight (reference the FARs; the list of items to identify the runway) then you must go around. If you do have the runway environment in sight, you can land. That is because the flight visibility was enough to identify the runway and land.

Now, if RVR is being reported (touchdown) and goes below mins while inside the FAF, you must go-around. RVR is controlling.

So, to the original poster of this thread... let's say you are flying an approach with a 200 ft decision height, with required visibility 1/2 or 2400 RVR.

Inside the FAF, tower says "visibility now 1/4 mile." You can continue to DH of 200 ft. Now, at 200 ft, you see the MALSR approach lights. "Continuing."

At 100 ft above TDZE, you see the HIRLs and centerline lights. You can land. You met the requirements to descend below DH, and met the requirements to land safely.

Change: Inside the FAF, tower says "touchdown RVR 1600, midfield 2400, rollout 2000." Can you land? No, you must go-around now, because RVR is controlling.


Hope this helps. If I am wrong on any of this, someone please feel free to correct me. But this is the way it has been explained to me many, many, many times.

 

[A Squared]

Now, if RVR is being reported (touchdown) and goes below mins while inside the FAF, you must go-around. RVR is controlling.

and:

Change: Inside the FAF, tower says "touchdown RVR 1600, midfield 2400, rollout 2000." Can you land? No, you must go-around now, because RVR is controlling..

Nope. this is a myth. An oft repeated myth, but a myth, nonetheless. RVR is not accorded any special status over and above reported ground visibility by the regulations. Yeah, RVR is more elaborately measured than visibility, and it's probably more reliable and accurate than reported ground visibility. But, like ground vis, RVR is NOT flight vis and flight vis is what counts.

Flight visibility is controlling. Period 91.175 and 121.651 both specify "flight visibility" If it *says* flight visibility, it *means* flight visibility. The FAA has made thier position on the issue clear. Read the legal interpretation below.

Note, though, that the FAA isn't going to unconditionally accept your determination of flight visibility. If you landed with 400 RVR and claimed that you had adequate flight vis, you might be subject to enforcement by a very skeptical FAA.






FAA Legal Interpretation:
March 10, 1986

Mr. Larry K. Johnson

Dear Mr. Johnson:
This is in response to your letter of February 6 requesting an interpretation of Federal Aviation Regulation (FAR) Part 91, Section 91.116.

Specifically, you request clarification of the term "flight visibility" in connection with the requirement in FAR 91.116(c) that an aircraft not be operated below a published decision height or minimum descent altitude if the flight visibility is less than the visibility prescribed in the standard instrument approach procedure being used. The question arises as to whether descent below the DH or MDA can be made when the runway visual range (RVR) is reported at less than the published minimum RVR for the approach but the flight visibility is greater than that minimum.

The flight visibility is controlling. If the flight visibility exceeds the published minimum for the approach, than the pilot may proceed as long as the other requirements of paragraph 91.116(c) are met regardless of the reported RVR. The National Transportation Safety Board (NTSB) has upheld this interpretation in several enforcement cases. However, the pilot's judgment of flight visibility is not necessarily conclusive if there is a question as to the actual flight visibility conditions at the time of the approach. Reported visibility and other evidence of record may be considered by the Federal Aviation Administration and the NTSB in determining the actual flight visibility.


Enforcement action would be taken only in those cases in which the pilot could not reasonably conclude that flight visibility was at or above approach minimums, but the pilot nevertheless proceeded to land or descent below DH or MDA.

Sincerely, David L. Bennett
Manager, Airspace and Air Traffic Law Branch
Regulations and Enforcement Division

 

[FlyChicaga]

Hmm, interesting. I stand corrected. This is contrary to what is being taught in many training programs.

 

[A Squared]

This is contrary to what is being taught in many training programs.

Yeah, no doubt. That's what I meant by oft repeated myth. I'd heard it quite a bit too, and was about 75% convinced it was true, until I saw the legal interpretation.

 

[GCD]

In 1999, a Boeing 727 crew was violated for landing when the reported RVR was less than needed for their operation.


I posed this question to a good friend of mine who is an FAA Ops Inspector. He said that Inspectors argued this question all day at his FSDO. In the end they all agreed. If 1800 RVR is needed, but 1600 RVR was reported, then they would go with the PIC's judgement. However, if the reported RVR was less, say 1000 RVR, and something brought it to their attention, questions would be asked.

 

[Booster]

Are the FAR's Muddy!

91.175 Takeoff and landing under IFR.

(a) Instrument approaches to civil airports.

Unless otherwise authorized by the Administrator, when an instrument letdown to a civil airport is necessary, each person operating an aircraft, except a military aircraft of the United States, shall use a standard instrument approach procedure prescribed for the airport in part 97 of this chapter.

(b) Authorized DH or MDA. For the purpose of this section, when the approach procedure being used provides for and requires the use of a DH or MDA, the authorized DH or MDA is the highest of the following:

(1) The DH or MDA prescribed by the approach procedure.

(2) The DH or MDA prescribed for the pilot in command.

(3) The DH or MDA for which the aircraft is equipped.

(c) Operation below DH or MDA. Where a DH or MDA is applicable, 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 -

(1) The aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under part 121 or part 135 unless that descent rate will allow touchdown to occur within the touchdown zone of the runway of intended landing;

(2) The flight visibility is not less than the visibility prescribed in the standard instrument approach being used

125.381 Takeoff and landing weather minimums: IFR.

(b) Except as provided in paragraph (c) of this section, no pilot may execute an instrument approach procedure if the latest reported visibility is less than the landing minimums specified in the certificate holder's operations specifications.

Pilot/Controller Glossary

Landing Minimums - The minimum visibility prescribed for landing a civil aircraft while using an instrument approach procedure. The minimum applies with other limitations set forth in FAR Part 91 with respect to the Minimum Descent Altitude (MDA) or Decision Height (DH) prescribed in the instrument approach procedures as follows:

a.Straight-in landing minimums. A statement of MDA and visibility, or DH and visibility, required for a straight-in landing on a specified runway

Dictionary of Aero Terms

landing minimums, IFR (aircraft operation). The minimum visibility prescribed for landing a civil aircraft while using an instrument approach procedure. The minimums apply with other limitations set forth in 14 CFR Part 91 with respect to the minimum descent altitude (MDA) or decision height (DH) prescribed in the instrument approach procedures.

121.567 Instrument approach procedures and IFR landing minimums.
No person may make an instrument approach at an airport except in accordance with IFR weather minimums and instrument approach procedures set forth in the certificate holder's operations specifications.


My company allows us to begin an instrument approach if the VIS is at or better than the required published vis for that approach. We may continue the approach if VIS is reported as below the min required if we have already passed the FAF.