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-   -   Unpublished let-downs (https://www.pprune.org/private-flying/589808-unpublished-let-downs.html)

TooL8 19th Jan 2017 13:36

Unpublished let-downs
I'd like to start up a discussion on unpublished or 'home-grown' instrument let-downs.

Many of uf with IR or IR(R) find ourselves at local airfields with no published procedures. Arriving VMC on top with 8/8 below, a diversion to the nearest published procedure is obviously the 'correct' exam answer; but real-world, many of us will letdown to MSA on a VOR radial or GPS position (RAIM checked;)).

The recent fatality in Oxofdshire thread raises doubt as to the long term safety of this practice (it's mentioned in one of the threads). Can I question this? If accurately flown, with stations tuned, identified and 'flags away' and or GPS RAIM checked, what do we think are the risks?

Collision with other acft is a given. Would a traffic service be mitigate risk sufficiently? What does everyone think?

chevvron 19th Jan 2017 13:45

When designing an iap, ICAO requires various 'extra' factors to be considered when calculating the minima eg terrain clearance in the missed approach area, type of approach and runway lighting.
Many people don't know this and just base their minima on the 'lowest possible' for that type of approach.
For instance, many years ago I got out the ICAO manual and tried to design an NDB/DME procedure for runway 26 (now runway 25) at Blackbushe. Although the lowest permissible minimum for this type of iap would be 250ft QFE, the minima taking all other required factors into account came to 620ft QFE.

piperboy84 19th Jan 2017 14:10

I designed a "homemade" approach to my grass strip a few years back and flew it many times in VFR conditions, it was loaded on the panel mounted Garmin 496 with an external antenna so was pretty much rock solid. I kept refining then flying it always in VFR conditions over and over again till I had it nailed. Then the first day I went to try it in actual I was lined up 15 miles out on final at 4500 ft with the overcast between 1000 msl (750 AGL) topping out at 3000 and I started my descent for the soup. Just before entering i thought to myself f:mad:K this! powered up, turned south and headed 15 miles out to sea, picked up the Leuchars ILS and rode it down to VFR then flew back to my strip low level with plenty of head height/ground clearance from the ceiling.

Bottom line, it just ain't worth it.

Edit to add, I had the GPS with the signal and accuracy alarms set which would pipe an audible warning thru the audio panel to my headset. Still didn't feel confident

2 sheds 19th Jan 2017 14:13

You mention descent to MSA, which is fine, but you start the thread by referring to "unpublished or 'home-grown' instrument let-downs" which is a worry and could subsequently comprise part of an accident report.
Of course a Traffic Service would reduce the risk - but whether "sufficiently", that's your guess! By the very nature of what you are suggesting, at or around MSA (and it depends on the ATC unit's agreed "terrain safe level") you would find yourself being told "take own terrain clearance" - and also probably "reduced traffic information due to the limits of surveillance coverage."

2 s

sapperkenno 19th Jan 2017 14:39

Having flown a lot in and around Phoenix, Arizona, the prevalence of GPS/RNAV approaches (even 10 years ago), in a state well known for hardly ever having poor weather... almost every airfield with any sort of published approaches had one. From the smallest private air park to almost every municipal airfield as well as Sky Harbor.
I won't go on too much about training and being tested on GPS approaches during my US instrument rating, and more recently doing my EASA CBIR have to re-learn 20 ways to skin a cat and fly a textbook NDB hold - with no mention of GPS, and this despite training on a G1000. :ugh:
Now having worked as an FI in the northern U.K. for the last 4 years, I know only of Blackpool, and I think Manchester in this neck of the woods that have these procedures... Sherburn and Leeds East keep promising their's, which never seem to come.

The problem wouldn't happen to be our CAA now would it, and the massive costs to approve such approaches?

To answer your "what does everyone think" I'd say the problem lies with our legislators and their archaic ideas of what should be trained/tested for the instrument rating, and how much they charge to approve such procedures... much easier to prohibit GA further, and stick to systems which in all fairness aren't broke, so don't really need fixing! As long as CAT is looked after, and only a few GA pilots are killing themselves every few years, and not wiping out loads of people on the ground, nothing will change.

ChickenHouse 19th Jan 2017 14:50

Nice question, but when I started thinking, I immediately felt a lack of information!

Anybody here firm on details how "published" let-downs are done?

What are the steps to let us assign a quality flag and what do we exactly gain on the "this is a published approach"?

Yes, a clerk at his table and a crew in an official aircraft, measuring approaches twice a year in VMC, do have the responsibility to do their job right, but they also have the "get out of jail for free card" when just following written procedures.

But no, on the other side I have the ultimate accountability sitting left seat and if I fail, I die. If I build a personal let-down based on daily ops at a specific airfield, will it do better or worse than the published, for me and/or others using it?

What does this tell us about the potential quality of approach let-downs? I.e. if I know I have to cut that corner 10 degrees left, because I always have the tendency to do this and that, I gain a personal let-down for my skills. Does a "published" approach gain additional safety margin, because the measure will be "the average pilot"?

The longer I think about, the more complicated the original question gets.

oggers 19th Jan 2017 15:01

Descending to safety altitude (and not below) for a look is okay. Continuing below safety altitude on an unpublished home grown procedure is not something that anyone should be doing except as a last resort in an emergency. It should certainly not be done to avoid:

a diversion to the nearest published procedure

JW411 19th Jan 2017 15:17

Many years ago, I used to operate from a strip in Germany with no aids. A friend devised a home-made letdown based on a nearby VOR/DME. I seem to remember that he had worked out that an MDH of 500 feet AGL was safe. One day he came down through the murk and was delighted to see the strip nicely on the nose when he broke out at around 600 feet. His joy was short lived; seconds later two NATO F-104s rocketed underneath him on a reciprocal track! An immediate landing for an underpants change became his first priority.

Sillert,V.I. 19th Jan 2017 16:19

Originally Posted by TooL8 (Post 9646397)
I'd like to start up a discussion on unpublished or 'home-grown' instrument let-downs.

This is something which I have done a long time ago, but wouldn't do now.

Descent to MSA requires mitigating only the air-to-air collision risk and a basic radar service should take care of that. Descent below MSA requires mitigating the CFIT risk and IMO that is far more likely to happen in the real world.

Unfortunately to get the job done, descent below MSA is usually required and although we all know this is something thou shalt not do, I suspect the practice sometimes happens.

The temptation may be greater today than it was when I started flying in the '80's; there are fewer viable, affordable, GA friendly options for using airfields with published procedures now.

Availability of moving-map displays, both certified and otherwise, may also increase temptation and risk, without properly mitigating the inherent danger.

I wouldn't wish to see anything posted here which could be construed by my younger self as encoraging this practice.

In short, please don't do it. Your life is precious and you have only one, something I have become increasingly aware of with advancing years.

Stay safe out there.

2 sheds 19th Jan 2017 18:06

a basic radar service should take care of that

Presumably you mean a Traffic Service (in the UK) ? There has never been a "Basic Radar Service". But see #5 above.

2 s

alex90 19th Jan 2017 18:44

I was under the impression that Instrument Flight Procedures for RNAV/GNSS approaches were now considerably more accessible than they once were. I have been told that having a GNSS approach would considerably reduce the cost of running and maintaining real beacons, but more importantly, that these approaches were not as expensive as first thought to get approved.

I remember being given a ballpark (which stayed in my mind for some time), in the order of 40-50k for the airport survey, IFP GNSS design, UK CAA paperwork, validation flight, risk assessment, proposition for airspace changes, CAA charges and risk assessments. For 2 reciprocating runways.

I assume that these costs vary wildly depending on many many factors of course, but at a cost of 50k the question is: How much do you value using an airport / aerodrome on days where clouds are at or below MSA - if this is of high value - how much do you value your life? And how much have you spent on your aeroplane? Is it worth invalidating your insurance to attempt the descent? Surely just a small group of owners could easily get together at smaller fields to purchase a published instrument approach - maybe even striking a deal with the airfield re: landing fees being waved or IFR approach fees being paid back to them when it has been used by others... etc...

I would not descend below MSA, unless in an emergency of course (ie: engine failure) if I was not on an approach! I would seriously question the sanity of anyone who would!!

Pittsextra 19th Jan 2017 19:01

descent below MSA on approach or at some other time surely just requires one to be certain of ones position? If you can achieve that then it becomes less astonishing doesn't it?

Gertrude the Wombat 19th Jan 2017 19:22

surely just requires one to be certain of ones position
... which you never can be, exactly, which is why approaches need designing to acceptable inaccuracy limits, with get-outs when things go pear-shaped, ect ect ...

Pittsextra 19th Jan 2017 19:30

sure no truck with that argument but to be clear if the issue is positional error and as you rightly say an acceptable margin then that can/could be done regardless of someone in an office deciding so. An individual could be quite capable of achieving something quite flyable.

piperboy84 19th Jan 2017 19:59

1 Attachment(s)
I'd imagine a lot more folks will be trying descent below MSA now that Garmin is selling this VFR GPS for under $1000 with synthetic vision

Attachment 1633


Downwind.Maddl-Land 21st Jan 2017 17:26

A similar subject was aired on the Flyer forum a while back. Taking and regurgitating one post probably explains why RNAV IAPs haven't been widely adopted in the UK, yet:

Backs of fag packets are not a UK CAA approved design tool.

The protection areas applicable to IFPs (and they are different for each type of IFP - but someone, somewhere, in ICAO did a LOT of clever mathematics to work out what those criteria might be) take into account many variables - including the concept, for example, of 'spiral winds' in which the nominal track of the aircraft is assumed to be adversely affected by the 'worst case' winds applicable to the calculated altitude throughout the turn. Consequently, the areas to be examined for obstacles would surprise you; they are far larger than you would imagine. Unfortunately, hard past experience has shown that the protection areas are not unduly conservative.

However, that is far from the whole story; its the allied: satellite coverage prediction report, increased (new?) CAP 232 survey requirements, safety case, ACP (that's the 'killer'), flight validation database production, flight validation plan, flight validation itself, etc that rack the price up. However, if you want your IAP to be Approved by SARG for promulgation in the AIP (discrete IAPs are no longer allowed) then that's what you have to do. And if you are a GA aerodrome with AFISOs, then you can include the CAP 1122 process costs as well!

The OP's post is probably a shining example of the old adage "a little knowledge can be dangerous."

tmmorris 21st Jan 2017 19:27

Isn't the problem with GNSS approaches in the U.K. more the requirement for approach trained controllers? Whereas the USA allows you to shoot an approach to a non-towered or closed airfield with a clearance from a controller many miles away, so the only cost is the survey and approach design (and many early GPS approaches were overlays).

Question: is ILS to an airfield nearby followed by 5-10nm of scud running safer than a home made approach flown using a proper approach certified GPS direct to the runway?

(And it's Alexander Pope - 'A little learning is a dangerous thing.')

tmmorris 21st Jan 2017 19:28

(PS I know that a nationalised ATC system and class E airspace everywhere 700 or 1200ft AGL are part of that US system too)

piperboy84 21st Jan 2017 19:38

Question: is ILS to an airfield nearby followed by 5-10nm of scud running safer than a home made approach flown using a proper approach certified GPS direct to the runway?
I'd rather take my chances shooting a homebrew approach with a non certified but extremley feature rich and flexible GPS like a Garmin 496 than I would with a cumbersome and purpose built certified unit like a 430. In fact I'd rather use an IPAD with SD than a 430.

Gertrude the Wombat 21st Jan 2017 20:14

(And it's Alexander Pope - 'A little learning is a dangerous thing.')
Yes. The "little knowledge" misquotation is wonderfully recursive.

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