Glideslope and visual approaches

[goldentrout]

What does FAR 121 say (if anything) about maintaining at or above the ILS glideslope during a visual approach? My IOE Captain seemed very concerned about going below the glideslope during the final 200' of the approach. My technique is to use the glideslope as a guide until around 500', and then I aim at the numbers for the rest of the approach. My reason for this is I see no reason to waste 1000-1200' feet of runway by following the glidesope all the way down, especially on a short and/or wet/icy runway where a little extra pavement might just make all the difference.

I'm only talking about visual approaches...certainly when doing an actual instrument approach, "ducking under" the glideslope is procedurely incorrect.

Thank you

 

[bigsky]

I dont have any references available at this moment- but the Fars somewhere say that if in a TURBINE aircraft and you have a vasi or an electronic glideslope available you must be on or above it from om to MM.
Also as the ac gets larger, ie.. wide body, the normal 2 bar vasi or glideslope may not even provide enough clearance over threshold. For example dc-10 requires a minimum threshold crossing height of 48 feet to ensure clearance..so as a rule ducking below gs is not recommended
I DO APPRECIATE your concern about wasting runway, however as a flight instuctor I once gave remedial training required by FAA to one of my clients.He had a mu-2 turboprop that had a landing gear collapse( no fault of his). However Faa made quite an issue about his landing on the numbers(his normal practice) vs remaining on the gs and landing in touchdown zone. because of that practice they gave him quite abit of harassment- he argued with them over it and eventually talked himself into a freecheck-ride with FAA--whatever you call those(768, 669 or whatever

 

[goldentrout]

I guess maybe the right answer is somewhere in-between. A pilot probably ought to maintain the VASI/glideslope unless, in his judgment, he's got a good reason not too...i.e. a situation where landing distance is critical...heavyweight on a short runway, poor braking action, etc.

Probably not a good practice to consistently duck below the glideslope in normal conditions with plenty of runway available, which is typically the case in the Dornier 328 that I fly.

 

[chperplt]

Your IOE instructor was right to jump on you for it.. 121 flying requires you to follow the glideslope or a visual reference if available all the way down. Do those things when you're on the line..

I can't imagine you're going to be taking your 328 into many tight spaces that would justify hitting the numbers.

 

[goldentrout]

Do you have a FAR reference? Everyone tells me "the FAR says," but I am unable to find it.

I agree there aren't many times when I need to aim (not hit, mind you...touchdown is typically 500-1000' beyond the aimpoint) the numbers, but there are times when it seems prudent...

1. Going into PHL on their N/S runway. It's only 5459'...and sometimes you need to fly faster than normal approach speed...since you have to time yourself to miss traffic landing east to west in front of you. If this runway was water logged or icy, I'd want every inch I could get.

2. Many airports have a long main runway, and a 5000-6000' shorter runway. If there's a 40 knot crosswind on the main runway, and the shorter runway is more aligned with the wind, I'm taking the shorter runway. Again, I see no reason to deliberately give-up 1000-1200' feet of runway on a 5000' runway...especially if I'm flying icing approach speeds (typically 10-15 knots faster than normal approach speeds).

I agree...in general I guess I should be at or above the VASI/glideslope on a visual approach, but I still think there are situations where judgment would dicatate a little shorter approach. Aiming at the numbers gets me on the ground safely 1000-1500' down the runway, instead of 2000-2500' by following the glideslope all the way down.

 

[Andy Neill]

91.129e2: " A large or turbine powered airplane approaching to land on a runway served by an instrument landing system (ILS), if the airplane is ILS equipped, shall fly that airplane at an altitude at or above the glide slope between the outer marker (or point of interception of glide slope, if compliance with the applicable distance from cloud criteria requires interception closer in) and the middle marker; and
3) An airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide path until a lower altitude is necessary for a safe landing."

These paragraphs apply to Operations in Class D Airspace.

 

[flydog]

The reference to Part 91 maintaing glideslpope to MM is correct. The same section also requires maintaining the VASI until safe landing is assured

Another thing to keep in mind is that an IAP is a clearance which must be adhered to. There technically is no such thing as "going visual" during an ILS. The glide slope must be followed to decision height regardless of weather conditions. After that if you want to yank the power back and dive at the numbers you may be using poor judgement but as far as the FARs go you may not be in jeopardy of being violated.

Just remember that 2 dots at the MM is about 35' low and the TCH in most cases is about 50'. All it takes is a little wind shear to put you in the approach lights

If you review your landing distance data you may find that it takes into account a 50' TCH and touchdown at the 1000' TDZ. Since you legally cant land unless you meet this factored landing distance when flying 121/135 then there should be no need to go below glide slope.

 

[flydog]

By the way just to add to Andy Neill's post.

The other paragraphs of Part 91 dealing with B,C, and E airspace also mention that all the requirments of Class D airspace apply to those classes of airspace to include maintaining the glideslope and VASI. The only airspace this is not required is in Class G

 

[avbug]

Goldentrout,

Under FAR 121 and 135, you are required to touchdown in the first third of the runway (touchdown zone). (14 CFR 91.175(c)(1)). It's tempting to look at the total useable landing distance, or runway length, when considering this requirment. It's tempting to determine if you can stop in the total landing distance, period.

However, if you look on the back of your runway diagram on your Jepp charts, under Additional Runway Information, you'll find the useable runway lengths after the threshold, and after flying the glideslop to touchdown. It is the useable distance after flying the runway to touchdown on the glideslope, that should concern you when flight planning to a given runway. If you believe that this distance is insufficient, then you should not use the runway, regardless of the total length.

Under TERPS, the obstacle clearance plane and distances are calculated for the actual glide slope to the DH. (now DA). Normally this is 200', except for Cat II and Cat III runways. The ILS is flight checked to this altititude. However, below that altitude, the obstacle clearance is only calculated, and the clearance and glide slope is NOT flight checked. Your only assurance of obstacle protection is to maintain the same glideslop angle protected on approach with the glide slope, OR, a visual approach slope indicator. (The VASI/PLASI, etc, will provide obstacle clearance, but may occur at a slightly different angle).

Aside from the other implications, there are legal considerations. To "duck under," you are operating outside the FAR, and you open yourself to liability if anything at all should happen (including enforcement action, for whatever reason). A possible scenario you may not have considered is damage to a trailing aircraft by wake turbulence. If that aircraft was damaged, regardless of what actually happened, you may find yourself on the losing end of a court case because you weren't where you were supposed to be. If that sounds far fetched, it's not. Your reasoning and the specifics may not be all that important.

Ask any carrier pilot about "spotting the deck," and see who thinks it's wise. Landing on such a critically short piece of real estate, the pilot is forced to fly a visual (and electronic) guidance all the way to touchdown. Even a few feet long or short can result in a bolter (missed approach), or a nasty sudden stop at the edge of the deck. Such a landing is a standard approach, in microcosm, with bigger penalties.

Finally, note that FAR 91.3 requires you to become familiar with the runway lengths before departure, as part of your preflight planning information. This information includes the useable runway lengths when landing beyond the glide slope. If this distance is insufficient, the runway is not acceptable for use. Factors such as a contaminated runway, aircraft weight, and density altitude at the destination will affect the ability to get stopped in this distance. You can always select another runway.

If dispatchers don't understand this, educate them.

 

[profile]

In addition to the requirement to maintain the g/s to the MM, you should also remember that if you start deviating from it after that point you are no longer flying a stabilized approach. The G/S is taking you to the point you need to be aiming at. Save the "land on the numbers" stuff for the little airplanes and bush pilots.