Go-NoGo Fog

[skidoo]

Bryon,

With a IFR WAAS GPS, the vast majority of LNAV only approaches have an advisory glidepath displayed. When this occurs, the annunciation is LNAV+V, with the +V indicating the advisory nature of the glidepath indication. The advisory glidepath may be followed in the same was as a LPV or LNAV/VNAV, but the altimeter still must be used to determine any minimum altitude or MDA. Since any descent rate up to a 7 to 1 slope may be used by the pilot on a LNAV approach, following the glidepath is acceptable between minimum altitudes as it is well shallower than the 7 to 1 slope. The advisory glidepath is considered as merely an aid to establish a stabilized approach to the MDA, and has no consideration what so ever for obstacle clearance once you are below the MDA.

There is more consideration given for obstacles on a LPV and LNAV/VNAV, but the path below the DA does not have to be obstacle free. If I am interpreting the TERPS correctly, obstacles on a vertically guided approach such as a LPV must not be higher than 2/3 of the glidepath angle, starting from the threshold and continuing to the DA. So with a 3 degree glideslope, obstacles can exist as long as they don't exceed a 2 degree slope. There are some fine details that I am ignoring for this discussion, such as the TCH, curvature of the earth, ... .

In this case, there is only a GPS approach available and its MDA is 1600 or 1400 ft AGL depending on direction. Either way, the top of the fog layer is 800 ft AGL. So, One way back in is over water where the threshold is perhaps <500 ft from the shoreline. So, you can know there are no obstacles down to a hundred feet or so. That is if you can skew the glide path one direction because there are hills on the other side which is probably the reason for the high MDA. But, if the engine quits and you end up in the water, we would die anyway of hypothermia (water near 33F.)

 

[Jay Honeck]

This is a fascinating topic -- thanks for starting it. IMHO it clearly shows the delineation of pilots into two groups:

1. Those who feel completely comfortable flying instruments in GA, piston-engined, SEL airplanes.

2. Those who do not.

Mary and I are in Group 2, but I know plenty of folks in Group 1 who would depart into this thread's fog-bound scenario without a second thought. Most of them are still alive.

Given the number of mechanical issues we've experienced in our 2000+ hours aloft, I'm always torn between admiring these folks, and believing them to be insane. There just isn't anywhere I want to be THAT badly.

 

[N2124v]

Hmmm, wouldn't these be a scenario where a chute would be a nice option?

Fire away....

 

[John Collins]

In this case, there is only a GPS approach available and its MDA is 1600 or 1400 ft AGL depending on direction. Either way, the top of the fog layer is 800 ft AGL. So, One way back in is over water where the threshold is perhaps <500 ft from the shoreline. So, you can know there are no obstacles down to a hundred feet or so. That is if you can skew the glide path one direction because there are hills on the other side which is probably the reason for the high MDA. But, if the engine quits and you end up in the water, we would die anyway of hypothermia (water near 33F.)

I can't find where you mentioned the airport ID, but from the picture of Ronan, MT there is an airport 7S0 with two RNAV approaches, but the LNAV MDH is actually quite low, only 319 AGL for the RNAV (GPS) RWY 34 only 374 AGL for the RNAV (GPS) RWY 16 approach which the final approach course is over the lake. I don't get the 1600 foot MDH aspect. From your first picture, it appears that you could easily find a field to make an off airport landing in. What is the airport ID you are talking about?

 

[Ghery]

No go for me. If there is a problem that requires returning to the departure airport, how are you going to do it? A complete engine failure and how are you going to pick out a place to put it down if you can't see through the fog? IR or not, I would not be comfortable pushing the throttle forward. And, yes, I fly out of a field with a reputation for being a fog hole.

 

[skidoo]

I can't find where you mentioned the airport ID, but from the picture of Ronan, MT there is an airport 7S0 with two RNAV approaches, but the LNAV MDH is actually quite low, only 319 AGL for the RNAV (GPS) RWY 34 only 374 AGL for the RNAV (GPS) RWY 16 approach which the final approach course is over the lake. I don't get the 1600 foot MDH aspect. From your first picture, it appears that you could easily find a field to make an off airport landing in. What is the airport ID you are talking about?

You are right, I didn't mention the ID. But for your ref, here is what I was thinking of http://airnav.com/airport/8s1 . 7S0 is nearby and has lower minimums. But, as you can see, it had some fog as well. I would just consider that one an en-route airport all of which would be expected to be covered for 50 miles.

 

[skidoo]

No go for me. If there is a problem that requires returning to the departure airport, how are you going to do it? A complete engine failure and how are you going to pick out a place to put it down if you can't see through the fog? IR or not, I would not be comfortable pushing the throttle forward. And, yes, I fly out of a field with a reputation for being a fog hole.

Great response! I would like to use this in a follow up thread shortly...

 

[John Collins]

You are right, I didn't mention the ID. But for your ref, here is what I was thinking of http://airnav.com/airport/8s1 . 7S0 is nearby and has lower minimums. But, as you can see, it had some fog as well. I would just consider that one an en-route airport all of which would be expected to be covered for 50 miles.

Thanks for the ID. These two straight in approaches don't have the advisory vertical guidance anyway. Also the approach over the lake has a significant obstacle on the final approach course as well. These are interesting approaches, I will add them to my set of approaches.

 

[SteveinNC]

Very easy decision for me (certainly only consider it IFR)...if I can't get back down on some kind of approach either at the airport I took off from, or one that's REALLY close by..I simply don't launch.

So, a no go for me.

 

[somorris]

Can one safely and confidently depart in 1/4 mile visibility, and
Is one's risk of needing a return to the departure airport low enough to make it a go decision?

I guess I am a lot more risk averse than some here. If I can't get back down to the departure airport, I would not go unless there was a clear alternate very, very close (not 21 miles as you said).

 

[Fearless Tower]

Probably depends on the mission requirement, pax or not, and location.

So far in my flying, I have done two zero - zero (indefinite ceiling and less than 1/4 vis) takeoffs from fields that I could not have returned to. In both cases, I had no passengers onboard and I'm not so sure I would have done them with pax onboard.

In the first case, it was an early morning departure from McCambell - Ingleside Texas (TFP) in a single engine. There was ground fog at the airport, but clear a few miles away along the route. It was not very thick vertically - you could see some stars peeping through, but definitely forward vis was nill. I am instrument rated but took off VFR since it was G airspace and I knew that I would be in VMC well before I entered any controlled airspace. I also took off knowing that the terrain around the airport was all FLAT and plowed farmland or light brush so I had a plan for engine failure on departure.

The second zero-zero departure was IFR in a twin from San Diego Montgomery (MYF). Same ground conditions, but thicker layer. Obviously no returning to the departure field, but I knew that the ceiling at SAN (6 nm SW) was above the DA on the ILS at runway 9 (I was departing 28R at KMYF, so I briefed myself that IF I had a problem on the initial climb, I would declare an emergency and request a vector for the ILS to SAN. I also had the SAN ILS 9 approach plate out on the yoke and Nav radios set up with the appropriate frequencies.