FAR 23.75 landing distance- descent angle greater than 5.2%

[HarleyD]

FAR 23.75 excerpt:

(a) A steady approach at not less
than VREF, determined in accordance
with § 23.73 (a), (b), or (c), as appropriate,
must be maintained down to the
50 foot height and—
(1) The steady approach must be at a
gradient of descent not greater than 5.2
percent (3 degrees) down to the 50-foot
height.
(2) In addition, an applicant may
demonstrate by tests that a maximum
steady approach gradient steeper than
5.2 percent, down to the 50-foot height,
is safe. The gradient must be established
as an operating limitation and
the information necessary to display
the gradient must be available to the
pilot by an appropriate instrument.

My first question is regarding the required 'appropriate instrument/ indicator" that is mandated for any approach angle in excess of 3 degrees?. An approach angle indicator is not a typical instrument found in a GA aircraft, especially a small one such as would be used for operations where a steep approach profile and short landing distance may be required.

I am also a little confused as to intent of this paragraph, as the fact that whilst when making an ILS and utilizing the GS indicator a pilot has an indicator that shows a 3 degree (5.2 % ) approach, but if he is not so equipped at an airport without a serviceable ILS there is no indicator for the 3 degree profile, so why is one needed for approach angles in excess of this gradient?

If a light SE aircraft is intended for use in 'bush' environment, the shortest landing distance possible should be available as a procedure for the pilot (within the constraints of AC 23-8C). this would normally entail a steeper than 3 degree approach, though without excessive ROD, This aircraft would likely NOT have an approach gradient indicator for either the standard gradient or for angles greater than this.

HD

[stressmerchant]

I gather the intent of the rule is to standardise the techniques for determining landing distances. Prior to the standard slope manufacturers could use steep approaches to limit the horizontal distance, which resulted in one or two really heavy landings during the certification phase. Aside from the aircraft damage, it was unlikely that these distances would be realistic for operators. Using a normal ILS approach angle makes sense as a "common playing field" approach, even if the actual approach is to be made without any form of ILS

The "instrument" was probably not defined deliberately. This left the door open for new developments that were then on the horizon - including MLS.

[safetypee]

Can’t speak for part 23 directly, but the text (intent) appears similar to part 25.
Whilst there have been standardisation efforts in the determination of landing distance tests, the steep approach requirements originated from a FAA category “STOL” (circa 1980) which applied primarily to operations with specialist aircraft.
The essence of the STOL advice was incorporated in a certification working paper used primarily by the Canadians (DHC) and then the UK (special conditions) before this document was incorporated in JAR-OPS and CS25 (circa 1995). Harmonisation with the FAA turns full circle to part 25.

The steep approach requirements allows the use of 35ft screen height vice 50ft for approaches above 4.5 deg, but is was judged that part 25 aircraft required more precise ‘external’ vertical guidance (collision risk / obstacle clearance), hence the need for a precision glideslope - electronic ILS/MLS or if visual PAPI or similar. IIRC neither GPS nor VASI are acceptable.

Thus aircraft used for commercial operations require an external glideslope and instrument display, particularly so in the confines of the quoted regulation – ‘landing distance’; but perhaps GA operations no so.
Part 25 steep approach has other airborne performance demonstrations and handling requirements.
Some aircraft are able to claim landing distance credit from the geometric advantage of a low screen height and a corresponding low flare height, also that if manufacturers had demonstrated ‘3 deg performance’ from a steeper approach, then these actual distances could offset some of the additional margin requirements.

[HarleyD]

Stressmerchant:

Yes clearly this is the intent, but it has a greater application for commuter category aircraft and for Part 25 of course.

The two specific issues of relevance for a small SE ACFT is that it essentially precludes an approach more 'normal' for this class of aircraft, i.e. a low power semi glide type of approach where an approach angle steeper than 3 degrees is typical. also these aircraft in the vast majority of operations are not on a GS commanded approach.

So, how is such an aircraft provided with an indicator to show 'correct' 3 degree approach when NOT operating on a GS? surely this is just as important as determining ANY approach angle that may be specified in the AFM, but there is no requirement stated for an instrument to provide such an indication, ONLY required if approach gradient is in excess of this value, say 5.3%??

Small aircraft with low Vref values are significantly more affected by environmental effects such as wind and this can dramatically affect the actual approach angle flown through the air, as opposed to a gradient referenced from the 50ft screen relative to earth. the pilot is required to make significant approach profile judgment and corrective inputs in any case, and a GS is not always availabel, or even used when it is available.

If the intent is to provide for obstacle clearance, a steeper approach will provide an additional margin, without the drama associated with heavy landings.

The requirement is that the landing procedure after passing the screen to touch down, must not require any 'particularly skillful or abrupt maneuvers' and also should include a 'smooth flare' and using 'normal pilot' application of controls and delays, and that at least 6 landing be demonstrated to this standard, using the same wheels brakes and tires. this should show the spirit to NOT include any such heavy arrivals as a matter of course.

If the applicant wishes to use a gradient in steeper than three degrees, this is an additional chart or table that is to be included in the AFM, as well as the three degree one(s). fair call BUT why the requirement for an 'instrument' for a visual approach, but not necessarily for an instrument procedure (other than ILS) some, possibly many (in the USA) small aircraft have an ILS, very few single engine pistons have MWLS, and are often flying Visual or IFR procedures into airports that do not have either ground based systems, so WHY the requirement for an instrument for a 4 degree approach but NOT for a 3 degree approach. this appears contradictory.

Safetypee:

PAPI or any other ground based visual system is not in compliance with the requirement:

information necessary to display
the gradient must be available to the
pilot by an appropriate instrument.back to my original question, is ther a simple way to comply with this requirement, and nominate a steeper approach, of course while still complying with the balance of the compliance requirements in order to provide more applicable landing distance charts for a small SE aircraft ( 10 seats or less)?

If the Beaver was certified today, how could it comply with this requirement? It is a mainstay in the bush pilot operations and not one where every approach is a three degree gradient. This is what made it so successful. The current requirements are compromising the manufacturers ability to compete with old (even antique) aircraft in a realistic manner. requiring a nominated 'procedure' for a steeper approach and publishing it in the AFM is a relatively simple task and not one that need include ant abnormal and/or abrupt maneuvering, but has the potential to reduce landing distances sufficiently to enable an operator to plan loads into airstrips that would otherwise be acceptable to older aircraft, but not acceptable for newly certified ones. this is counterproductive, and does not encourage replacement of an aging fleet with newer designed, safer aircraft.

HD

[djpil]

I haven't tackled this reqt but I have produced AFM landing charts with different approaches specified so just a thought.
An instrument displaying gradient is not required.
You need an instrument providing information.
From which gradient is displayed - debate how - and the gradient is an operating limitation.

VSI and ASI provide information.
Provide AFM chart with the gradient limitations shown as ROD for the appliable approach speed. Toss in altitude and temperature in the chart.

Fly the aeroplane at the specified approach speed. Set descent profile within ROD parameters from the chart.

I could point at that chart and state that the pilot is provided with instruments which provide information enabling the chart to display the approach gradient.

If it was me, I'd be looking at AFMs to see what others may have done.