Less than 3° glideslope.
 

I was wondering what's the advantage/purpose of having a less then 3° ILS GS or PAPI. I have seen a PAPI of 2.7° in Zakinthos (LGZA) RWY34 and ILS G/S and PAPI in Istanbul Ataturk (LTBA) I believe RWY36R.

Usually military traffic, lots of excess thrust and not a lot of drag so flat approach works well. Never been to those airfields though so unsure if that fits.

maybe it's got something to do with the view of the runway you get flying some military ac with different aerodynamic characteristics concerning AOA etc or maybe a flatter approach is better for them because they barely flare on landing:confused: brindisi also has one of those 2,5° ILS'es....

The 3 degree powered approach has become a standard arising from many years of experience. Any skilfull pilot will readily recognise a small deviation from 3 degrees and endeavour to regain that angle. It sets up a pilot with a standard view of a runway and an established expectation of all of those intuitive/trained control inputs to achieve a smooth touchdown and rollout.

The only acceptable variation should be related to terrain clearance during an instrument approach. Can't think of a good reason for having an approach angle less than 3 degrees unless it relates to spool up time for particular engines.

Caution should be advised using glidepaths of less than 3 degrees, with large wide-body aircraft.
The threshold crossing height will be lower, and the main landing gear might well be rather close to the frangible approach light bits, as a result.
Automatic approach/land ops especially.

For example, the standard body L1011 is limited to a minimum TCH of 42 feet, for automatic A/L ops.

And yes, before anyone asks, these aeroplanes still roll-on smoothly during these automatic approach/land ops.
Nothing finer.
Now, back to normal programming...;)

Agree with first reply, it is a miltary requirement for older generation fighter jets. I believe the UK RAF have/had some 2.5 degree ones that have progressively been updated to 3 degrees as the older fighters have retired or been replaced by ones capable of using a 3 degree glideslope.

Hmm I wonder why Luqa, Malta (LMML) has a 2.85 degree glide on Rwy14. Seems a strange number to pick. Sure enough this used to be RAF Luqa, but I think before the times of military jet aircraft. The approach takes you straight over the heart of the very built up island too... ??

The original ICAO standard ILS were based upon 2.5 to 2.75 degrees. This suited jets with slow spool up times and thus higher power needed to maintain the angle. VASIS were standardised to these angles back around the late Sixties.

Following usual pensioners noise complaints under glide paths the Sydney (Australia) runway 07 ILS was raised to three degrees giving extra height over the outer marker beacon. Raising the GP to three degrees made almost no difference to the noise foot-print but it made the local politicians happy that something had been done to counter the complaints. ICAO thought what a good idea and three degrees was adopted as the standard. In Australia it cost a lot of tax payers money to then standardise the remaining ILS and VASIS around this vast continent. I know because I flogged thousands of miles in the calibration flight DC3 as part of the job.

Again in the Fifties with introduction of fighter jets such as the Mirage into the Royal Australian Air Force the RAAF continued with a landing policy of touching down at the "piano keys" for fighters. This permitted more runway available in case of brake-parachute or aircraft brake failure due combat damage I suppose. I can only guess the current lower approach angle PAPI or VASIS mentioned in the first post is a relic of the old days where piano key touch down was considered the epitome of good airmanship (rightly or wrongly, depending on your viewpoint)

Thx for the replies. Zakinthos doesn't have any military activities anymore, but maybe the airport was built in the time that those GP's were the standard. The runway looks that old anyway :).

=5% gradient = 300ft/NM might be the reason

Vintage Prop?
 

I'd also have thought 411A would have noted that older piston aircraft have trouble with the 3deg slope; 2.5 is ideal. We cause quite a hassle if we have to do an ILS as the drag required slows us down [flap/gear limits.]

G'day ;)

Operated into Murcia last night. 2.8 degree G/P. Major Spanish military base; lots of fast jets and arrestor barriers at the end of each runway. Assumed the non standard G/P was due to the military hardware

Back in 1999 I flew into a few RAF bases in a Seneca. I asked wht they had PAPI's at 2.5deg.

I was told that it was in case a Tornado had a failure which meant that it would have its wing swung all the way back.

Seemed a reasonable explanation at the time.

It would also hold good for flapless landings.

The requirement on reference datum height remains unchanged as the GP angle changes. For CAT II/III you're guaranteed your 15 m at the threshold. For CAT I, it's "just" a recommendation from ICAO.

For short CAT I runways, the 15 meters shrink to 12.

In a friendly note, ICAO points out that the assumed maximum vertical distance from the GP antenna to the path of the lowest point of the aircraft is 5.8 meters. More than that - beware.

Reference: ICAO Annex X.

I visit an an airport where they have a 2.86 degree ILS at one end and a 3.25 at the other every once in a while. Multiple approaches on the 2.86 degree followed by a direct switch to the 3.25 makes for an interesting change of perspective!

My question: WHY are GS specified as X.X°, instead of x.x% ?? Percent, being a dimensionless number (albeit with a scalar of 100), would make approach calculations MUCH easier. This is one area in which the civil engineers have it right!

:rolleyes:

And what happens with the so called steep approaches? i.e. EGLC has GS 5.5º, or ENTC has GS 4.0º...

I've been trying to find a good definition of what you should consider as a steep approach without any luck. I just found on the brand new EU OPS that you have to establish procedures and get some kind of approval from the authorities for glide slopes > 4.5º. But once again, there is no definition available, unless you consider that if they speak for approvals if the GS > 4.5º, then no approval is required for GS < 4.5º... :{ But aircarft and crew qualification is required if you intend to use ENTC ILS to rwy 01 due to the steep angle!!!! What happens with those GS between that "standard" of 3º and 4.5º?

The main advantage of the 2.5 degree PAR glideslope at EGDG (Newquay) is the lower DA/DH, on occasions if needed we will use the 2.5 degree glide in preference to the 3 degree option.

This is for Turbo prop ops, no idea what the jet guys ops manuals say for EGDG.

2.05 Glide slope
 

I know of several ILS approaches with glide slopes other than the standard 3 degrees. The lowest of which I am aware is the ILS to runway 25 in Kodiak, Alaska (ADQ or PADQ) with a gs of 2.05 degrees. This is due to terrain, and the approach/missed approach criteria (TERPS). In addition, due to prevailing winds and often low ceilings, the advantage of a lower than standard GS is that it allows you to get to a lower altitude (below prevailing ceilings) before the missed approach point. Because of the terrain in Kodiak, the MAP is farther out.

In Ketchikan Alaska (KTN or PAKT) the glide slope for the ILS runway 11 is 3.61 degrees, although I believe both Kodiak and Ketchikan are tailored approaches sepcific to Alaska Airlines.

From an aircraft certification point of view, the threshold values for GS are 3.5 degrees and 4.5 degrees.

Below 3.5 degrees are considered "normal" approaches and the standard certification testing set out in, for example, AC25-7A, and the standard regulations in Part 25 (or whatever) are consider to apply and be sufficient.

Between 3.5 and 4.5 are considered to be "increased glideslope" approaches. For approval of an aircraft to conduct these approaches at an aircraft certification level, Transport Canada has a "Working Note" (I think it's #12, but I'll stand to be corrected there) which specifies certain avionics tests be performed. However, it is assumed that the basic handling and performance3 approval is adequate - unless the OEM is trying to claim credit for the steeper GS in some fashion (usually landing "air distance" or dispersion) in which case corresponding handling or performance tests are required.

Above 4.5 is considered a "steep approach"; the rules here are imposed by "Special Condition" on an aircraft - by - aircraft basis - generally a new Special Condition starts with the last one and makes minor changes if required. So there are an informal set of rules, but technically there is no standard rule. A full set of handling and performance trials are generally required, in addition to avionics type testing.

And in PANS-OPS, for Precision Approaches it states:

1.4.8.8.3.1.1 Procedures involving glide paths greater than 3.5° or any angle when the nominal rate of descent (Vat for the aircraft type x´ the sine of the glide path angle) exceeds 5 m/sec (1 000 ft/min), are non-standard. They require the following:
a) increase of height loss margin (which may be aircraft type specific);
b) adjustment of the origin of the missed approach surface;
c) adjustment of the slope of the W surface;
d) re-survey of obstacles; and
e) the application of related operational constraints.