IR Flying Into Private Strips?

[VFE]

Hello folks,

Something has been puzzling me of late and I was hoping someone here could put me out my misery! I live under the approach into a private strip. Now, on days like today I have seen the privately owned aircaft breaking cloud over my house (roughly 500ft) on approach to the strip.

What is bugging me is how can they be doing this as there is no NDB or ILS printed on the charts. Would it be possible to have a privately owned NDB that wouldn't be published? Who has an NDB at their private strip in this country?! The local military radar service is closed at weekends so he can't be getting a letdown from them. I am not trying to imply that they could be acting illegally but - and I guess I should know this seeing as I'm CPL/IR rated(!) - what I would like to know is thus:

Is it acceptable practice to decend through cloud on approach to an airstrip without ATC service and no landing aid such as NDB, ILS when the cloud base is roughly 500 ft and viz is less than 5000FT? My thoughts are that it is definately not but that's because at my level I wouldn't be happy doing it! Basically, they gotta be lining up on GPS, right? All very well if the altimeter is accurate eh? Would not puzzle me as much if my house wasn't under the approach path. :D

Answers, opinions?

Cheers,

VFE

[englishal]

Yea its ok ;)

As long as the GPS has been verified, and by that I would mean a procedure along the lines of:

Over fly the airfield at or above XYZ altitude, on a local altimeter setting from a nearby ATIS or facility. If GPS position agrees with ABC Radial and DME and / or DEF Radial and DME then head outbound start a procedure turn onto a pre-determined inbound course and descend to minimums on GPS.

I would be happy doing this if a) I had worked out the procedure beforehand and had produced a chart b) I had a good panel mounted GPS c) I had a way of verifying the GPS position i.e. VOR / NDB and / or DME

EA

[VFE]

Thanks Al,

What would minimums be then?

VFE.

[ShyTorque]

If not VFR / VMC then an aircraft must be flown iaw IFR rules.

Minimum altitiude IFR = 1000ft above highest obstacle within 5nm.

[VFE]

Cheers Shy,

Can you get GPS with altitude information now then?

Excuse the ignorance, not a big GPS man!

VFE.

[ShyTorque]

?? Not sure what you mean.

The altitude is referenced from the altimeter, using local QNH. If no local QNH was available at the location then regional QNH should be used.

[VFE]

What I mean is - the aircaft would be decending to a low altitude (incidentally, below IFR alt/NM limits) in milk on regional QNH without all the silly trimmings which lower pucker factor such as ATC radar coverage, GPWS, ILS app, NDB app etc...... which is still "fine" but only if the altimeter is correct. So how does one verify alt readout before commencing decent? And also, what would be the back-up on approach?

VFE.

[High Wing Drifter]

Can you get GPS with altitude information now then?
Some units will provide altitude information, so long as they can track at least four satalites. Problem is that the GPS altitude with probably be according to a WGS84 geoid model of the earth that probably may not predict sea level altitude accurately not to mention built in clock error and error due to the angle of the cut of the trajectories. All official GPS IFR approaches are, to my knowledge, according to pressure altitutude datums. For this reason I would consider conducting an IR approach on GPS altitude potentially lethal!

[VFE]

I should also say at this point, that I'm not looking to dob them in if it turns out to be hazardous. Obviously, a quiet word would be the prefered method so please do not worry about that.

VFE.

[englishal]

If not VFR / VMC then an aircraft must be flown iaw IFR rules.

The 1000' does not apply to take off or landing......

Many GPS units can accept a barometric pressure which they use to correct for GPS altitude errors. I would not use GPS altitude for terrain clearance, but use a local altimeter setting. As far as minimums go, its up to the individual and airfield location, 500' AGL may be suitable in a flat environment with no radio masts around.....

Cheers

[VFE]

Cheers Al! :D

You have been reliable in the past so I trust your words here. Sounds like there's serious scope for individual interpretation with this scenario - not sure I like that but hey ho......

VFE.

[jayteeto]

Since when have GPS approaches been legal?? You might be 'happy' to do them but whether it is approach/landing or not, I would check with the CAA whether they are allowed. I do not know the answer, but my belief was that this descent was not allowed. Please could someone official enlighten us???

[bookworm]

Published instrument approaches have no special status over something made up on the spot as far as current UK law goes. The closest official acknowledgement of this you'll find is in the long since abandoned proposal (http://www.caa.co.uk/srg/ats/default.asp?page=1587) to change the law to make it illegal to fly an unpublished IAP.

The fact that it is not illegal to fly an unpublished IAP does not, of course, preclude action against the pilot for endangerment if the risk management omissions warrant it.

[englishal]

Since when have GPS approaches been legal??

If a published IAP is in effect for an airfield, then you must use it, and not make your own up, and as far as I know there are no "legal" official GPS approaches in the UK. If no IAPs are published then you can create your own, using GPS, NDB, VOR, whatever.

Its up to the individual if they're prepared to risk life and limb on a GPS approach, I personnally would, and in the States GPS approaches are common place, in fact so common that airlines use them (heard Jet Blue being cleared for a GPS approach the other day).

Situational awareness and common sense is everything. As soon as you don't know where you are, its time to get out of there. A friend of mine flew into a mountain in IMC on a VOR/DME IAP, which had thay had a Garmin 430 may not have happened.....

:D

[VFE]

Last time I saw said aircraft do this in low viz/cloud base they were only slightly right of centre line.

Seems there's a loophole/grey area here - I would never had known half what Englishal has posted before today, quite suprised.

Seems it's a case of no rules against it so play on..... I wouldn't be too happy doing it, mind! At the end of the day it's an unmanned airstrip in countryside which means there could be wildlife on the runway which they would not spot until it was too late. No altitude minimums so no RVR regs then I take it? Bit slippery if you ask me.

Not to mention that the decent is commenced overhead congested town space.

VFE.

[Chilli Monster]

Its up to the individual if they're prepared to risk life and limb on a GPS approach, I personnally would, and in the States GPS approaches are common place, in fact so common that airlines use them (heard Jet Blue being cleared for a GPS approach the other day).
Let's just split these two comments here.

Article 40 and Rule 29 are (as is usual in the UK) very poorly written - the way they are at the moment don't specifically prohibit home made approaches for non public transport aircraft. 29 (a) should, in my opinion, be re-written along the along the lines of:

Without prejudice to the provisions of rule 5, in order to comply with the Instrument
Flight Rules an aircraft shall not fly at a height of less than 1000 feet above the highest
obstacle within a distance of 5 nautical miles of the aircraft unless:
(a) it is necessary for the aircraft to do so in order to take off or land;and the aircraft is following an approach procedure recognised by the authority
Such an amendment would make such activity illegal.

GPS approaches in the US are not 'home grown' affairs, but properly promulgated in current IAP charts - as such any similarity drawn between these and EA's home grown approaches that he would be happy to do are non-existent (before someone reading this thinks that's what's going on in the U.S)

[Keef]

Added to which, our GNS430 (all properly signed-off by the CAA for whatever a GNS430 can do) has a big placard on it GPS APPROACHES PROHIBITED.

ILS using the 430 (properly tuned) I'm happy with. To be honest, I'd rather do an NDB procedure with the GNS430 in GPS mode than using the ADF, but that ain't what the rulebook says.

[VFE]

Yeah Keef, NDB approaches amongst terrain always put me in mind of that childs game where you attempt to move the wire loop around the metal assualt course loop without touching it! Don't forget your lag now........ :uhoh:

VFE.

[tmmorris]

IMC rated pilot would still require 1800m vis, though. If he's IR then I suppose it's up to him.

Tim

[bookworm]

Not to mention that the decent is commenced overhead congested town space.

That makes it dodgy from a slightly different point of view. There's no exemption from the 1500 ft rule over a congested area for making an approach to an unlicensed airfield, whether VFR or IFR.

[FlyingForFun]

Out of interest, why has this topic been centred around GPS?

What's wrong with flying a radial from a nearby VOR, or a QDR from a nearby NDB? It's unlikely to be lined up with a runway at the private strip, but it can certainly be used to break cloud and perform a circle-to-land.

In fact, I used to use exactly this procedure to get back into my home airfield (licensed, but with no published instrument approaches) on nearly every lesson during my IMC rating training. The procedure involved an optional hold overhead a nearby NDB, then descending on an outbound track to hopefully break cloud (or, more likely, have your instructor remove the foggles) with the airfield positioned off to your left, just in the right place to join a left-hand circuit for any of the runways.

Bookworm... regarding your last post, I'm pretty certain (but stand to be corrected) that the 1500' rule applies to VFR flights only, so wouldn't apply in this case. However, the glide-clear rule would apply.

FFF
------------

[VFE]

That makes it dodgy from a slightly different point of view. There's no exemption from the 1500 ft rule over a congested area for making an approach to an unlicensed airfield, whether VFR or IFR.
This is what is happening. Which begged the question: could they have an unpublished NDB on their strip? Anyone know of any examples?

My house is part of the estate which is on the edge of the congested area (a city) and when the aircraft is above my house (and I mean DIRECTLY above it) it is circa 500FT. The only way they could avoid the congested area would be to do a circling approach. Very tight in the wx conditions I have observed them perform an approach in and as far as I have assertained though my ears - they don't do it - I could of course be wrong.

It is a MEP by the way......

VFE.

[bookworm]

Bookworm... regarding your last post, I'm pretty certain (but stand to be corrected) that the 1500' rule applies to VFR flights only, so wouldn't apply in this case. However, the glide-clear rule would apply.

There's nothing in Rule 5(1)(c)(i) (or any of the rest of the Low Flying Rule) that confines its application to VFR flights. It's, not hard to check it in CAP393.

Flights in accordance with normal aviation practice, for the purpose of taking off from, landing at or practising approaches to landing at a licensed aerodrome are exempted from Rule 5 in its entirety.

[smarthawke]

VFE

Just out of interest, you don't live in Peterborough do you? PM me if you like.

[drauk]

Better a GPS than tuning your ADF to Captial Gold in order to find Elstree, which I've seen done more than once.

[jezbowman]

I very much doubt they'll have a private NDB as to run it would require permission from the UK radio licensing authority. Futhermore the complications of ensuring continuity of service, range of service, etc. would make such an aid hard to gain CAA approval for. And why would you want CAA approval? Well, you can't even use MW radio stations for en route IFR flying, so you sure as hell can't use them for IAP's! So (and it's been said already) you can't go below 1000ft of the higest point 5nm from track without following an approved IAP using a CAA approved approach aid.

Correct me if I'm wrong, but the only correct thing to do here is route IFR to the nearest aerodrome offering an IAP, fly the IAP as a cloudbreak procedure and proceed VFR below the clag back to the strip.

Either way, buzzing around in cloud without a RIS/RAS is just stupid.

[Finals19]

This is very interesting, although I am a bit bewildered and rather concerned by some of the comments...??!!?

Are people saying that they are pretty much making up their own IFR approaches / let down procedures??? Seems tantamount to a potential "kiss-your-arse-goodbye" scenario, even with an IFR approved GPS. As for tracking out on a radial and letting down, then hoping to contact visual with the field, what kind of protection does this give you seeing as its an unofficial procedure (read: none?)???? And you had better be damn sure you know where you are regarding obs clearance minima etc.

Now I am not currently living in the UK (although have flown a fair bit there) but where I am flying, designer approaches are a huge no no. Granted there is a hell of a lot of terrain around which prohibits this, and also granted that GPS is used for many approaches where it can be deemed to be more accurate than a non precision NDB ... (see mountain effect on NDB pro's) This assumes RAIM availability at all times etc and a properly approved GPS, operating within published approach procedures.

I must admit once or twice coming into EGTR I considered tracking a radial due to poor viz, but the lack of protection from other traffic and lack of radar in the UK for a lot of IMC stuff makes it all a bit hairy doesn't it?????:bored:

[Dusty_B]

FFF:
Bookworm... regarding your last post, I'm pretty certain (but stand to be corrected) that the 1500' rule applies to VFR flights only, so wouldn't apply in this case. However, the glide-clear rule would apply.

Bookworm:
There's nothing in Rule 5(1)(c)(i) (or any of the rest of the Low Flying Rule) that confines its application to VFR flights. It's, not hard to check it in CAP393.

The 1500' Rule does NOT apply to IFR flights (including SVFR) in CONTROLLED airspace. Therefore, a MEP with positive climb capability could :uhoh: fly at 501' over a congested area!

[bookworm]

The 1500' Rule does NOT apply to IFR flights (including SVFR) in CONTROLLED airspace.

Where on earth do you get that from? Just cite the paragraph in the legislation that exempts IFR flights from the rule.

[FlyingForFun]

Jezbowman,So (and it's been said already) you can't go below 1000ft of the higest point 5nm from track without following an approved IAP using a CAA approved approach aidSorry, but that's not correct. You may go below 1000' when necessary for take-off or landing. There is nothing which says that the take-off or landing must follow an approved IAP when there is no published IAP for the airfield. In fact, bookworm has already posted a link to a document which proves this fact - a proposal (which has not been implemented) which suggests making it illegal, something which would not be possible if it were already illegal.

FFF
--------------

[drauk]

1500' over a built-up area DOES apply to IFR but not to SVFR if directed to fly lower - it doesn't absolve you of the need to be able to glide clear of the built-up area however. Reference is Rules of the Air rule 5 (for SVFR) and 5+29 (for IFR).

[UV]

I see, so were are IMC at 1000 ft and some people think its OK to continue the descent because we are going to land?? What cr"p.

Some scary advice here being offered here.

You cannot descend below the MSA unless you are VMC or in accordance with an approved instrument approach preceedure.

Period.

UV

[drauk]

UV, nobody advised doing it, just that the law didn't prohibit doing it. Of course, that doesn't mean it is a good idea necessarily.

[VFE]

Thanks for the replies folks. Keep them coming. Must confess to being slightly confused now though! Some people says it's perfectly okay, others say it is illegal and others say it is not illegal so is therefore acceptable! :\

VFE.

[Chilli Monster]

others say it is not illegal so is therefore acceptable!
I think you'll find a lot of people are saying it's not illegal but it's NOT really acceptable.

[PPRuNe Radar]

In the US, even your 'home made' approaches must be drawn up in accordance with Instrument Procedure design criteria (TERPS and ICAO SARPs) and approved for use by you by the FAA, although they will not necessarily be published for other users.

Furthermore, if using GPS, then the IAP is not legal unless the procedure is retrievable from the navigational database in the unit. GPS users are also not allowed to amend the procedure data or input their own, thus making true 'home made' procedures or those done on the hoof illegal.

That seems a very sensible way to operate. Alas, the UK has rules which only relate to aerodromes which have published IAPs :rolleyes:

As far as minimums go, its up to the individual and airfield location, 500' AGL may be suitable in a flat environment with no radio masts around.....

IFR height keeping tolerance is +/- 200' and obstacles up to 299' need not be promulgated. Doesn't make tootling around at 500' seem very clever does it ?? ;)

You cannot descend below the MSA unless you are VMC or in accordance with an approved instrument approach preceedure.

Period.

This is not what UK Law says. Most are of the sensible opinion it would not be wise, but that's different from it being illegal.


If the UK adopted the US system, I would support them :ok: - including GPS overlays.

[UV]

If one is not in VMC then one has to comply with the Instrument Flight Rules. Check?
This requires, interalia, 1000 ft above the highest object etc., etc.,
Therefore, you cannot "legally" do it. Check?
Wasnt there an article in GASIL a few months ago on this very subject?
UV

[PPRuNe Radar]

If one is not in VMC then one has to comply with the Instrument Flight Rules. Check?
Correct.
This requires, interalia, 1000 ft above the highest object etc., etc., Therefore, you cannot "legally" do it. Check?
Wrong.

ANO Section 2 The Rules of the Air Regulations 1996

Rule 29 Minimum Height Rule

Without prejudice to the provisions of Rule 5, in order to comply with the Instrument Flight Rules an aircraft shall not fly at a height of less than 1000' above the highest obstacle within a distance of 5 nautical miles from the aircraft unless:

(a) it is necessary for the aircraft to do so to take off or land,


The block text has been highlighted by myself and the remainder of the rule omitted as it is not relevant to this topic.

You can read for yourself here

UK Air Navigation Order (http://www.caa.co.uk/docs/33/CAP393.PDF)

[bookworm]

If the UK adopted the US system, I would support them - including GPS overlays.

I think you have to be careful about comparing the systems too closely.

The US has charted a far higher number of approaches, including GPS approaches, into smaller airports. It also has a system whereby access to the approach under IFR is controlled by Center with no requirement for the airport in question to be manned.

The UK sitaution is somewhat different. The CAA are not going to create approaches unless someone picks up the bill, and there's also a requirement for ATC at the airport. (The proposal I cited earlier was to revise this to a requirement for AFIS, but it's still a significant expense compared to the US situation where you can fly an approach at an airport without another human within 50 miles.)

The the US prohibition on home-made approaches is workable because it's enabled by the practicality. If such a regulation were adopted in the UK, the cost in terms of utility would be substantial. The quid pro quo for changing the law as proposed needs to be significantly more than just allowing IAPs under IFR with AFIS to make the cost-benefit stack up.

[englishal]

IFR height keeping tolerance is +/- 200' and obstacles up to 299' need not be promulgated. Doesn't make tootling around at 500' seem very clever does it ??

True when you put it like that, BUT I for one would never make use a "home made" approach somewhere I was not 100% sure of. Never done one, but know a few people who have used them. The difference is that these people fly daily from their strip and know the area inside and out in VMC.

I would rather make an approach which I knew and understood, in an area I was 100% sure of, than shoot a published IAP 20 miles away, then scud run "VFR" back to base, which some people have suggested would be the most sensible course of action.;)

[PPRuNe Radar]

I would go for the US system lock, stock, and barrel. Hence no comparison required ;)

The UK CAA no longer design IAPs anyway (unless contracted to for ££s), it is down to the operator of the airfield to have the work done and the CAA will then regulate the design.

In a similar way, whilst the FAA have done much work on formalising their existing IAPs, any new private ones can be drawn up by third party procedure design agencies (there are a few around) under contract to the individual who wants the procedure to use. Alternatively, there are various software packages around which can be used by those who have had appropriate training. Like the CAA, the FAA will then ensure it meets the IAP design requirements and grant approval.

I see nothing from a safety point of view to stop the UK introducing a requirement for all IAPs to be designed to meet ICAO and UK procedure design criteria and require CAA approval for use (either by way of publication in the AIP or by an individual approval). Allied to this, they could introduce a global minima to be used for any approach where no approved IAP exists.

Sure, it's going to cost some people money, but amending the legislation overall to allow a more flexible and safer operation of aircraft in IMC on IAPs which are proven to be safe without the need for ATC facilities could be seen as a fair trade off.

[IO540]

The best situation would be for every airfield to have a proper IAP with a navaid, but that's not going to happen because it would cost loads of money and the only "aviation" that matters to the authorities is the airlines and they fly only into big airports.

The next best thing and the smartest thing for the UK would be to draw up a GPS IAP for every airfield. That's not going to happen all the time the CAA pretends GPS doesn't exist. A whole generation of administrators (and pilots too) will need to retire first. Etc etc etc. There may also be fears re pilots doing them with handheld GPSs - in reality very few spamcan owners are going to spend the money on an IFR approved unit.

So the present situation where a pilot on a Private Flight can do a DIY cloud break is a freedom I would not like to have taken away.

[bad_influence]

The UK CAA no longer design IAPs anyway (unless contracted to for ££s), it is down to the operator of the airfield to have the work done and the CAA will then regulate the design.

Design of IAPs was put out to industry but, in usual CAA manner, they've changed their mind again and D.A.P. are responsible for this again.

[PPRuNe Radar]

Oh dear .... gonna take a while then !!!

[jezbowman]

englishal:
I would rather make an approach which I knew and understood, in an area I was 100% sure of, than shoot a published IAP 20 miles away, then scud run "VFR" back to base, which some people have suggested would be the most sensible course of action.

If it came to scud running then I'd rather fly the IAP 20 miles away and land, wait until it clears then transit VFR back to base. I'm still very wet behind the ears in terms of Instrument Flying, so perhaps in a couple of years I may also agree, for now I'm sticking with what I've been taught!


FFF:
I stand corrected. So not breaking the rules just, as I'm sure we all agree, not the cleverist of ideas.

[AlanM]

In the GASIL no 25 (Nov 02 so may be out of date)

USER DEFINED APPROACHES
a. Pilots have been known to produce and
follow their own approach procedures using
GPS information. This is potentially
dangerous. There is no ground based
confirmation of position and the risk of misentering
waypoints is high.

b. Furthermore, when flying towards a
waypoint in normal, en-route mode, the course
deviation indicator (CDI) normally indicates a
track error of 5nm at full-scale deflection (or 1
mile per ‘dot’). This is not accurate enough for
any final approach, and only changes when either the sensitivity is changed manually or the
aircraft is following a published and correctly
activated GPS approach contained in the
database. Changing sensitivity whilst on
approach is a hazardous dtraction.

c. Unless a published approach is activated,
the receiver’s RAIM function remains in enroute
mode (even if the CDI scaling is changed
manually) and there may be a position error of
up to 2 nautical miles before any RAIM alarm is
given.

d. User-defined approaches can be
dangerous and are not authorised.

13 SUMMARY
1. Accuracy is not guaranteed
2. Apparent accuracy does not mean reliability
3. Understand your own equipment.
4. Train before using it.
5. Use standard settings and check lists.
6. Flight plan normally before loading a route.
7. Check the route before flight.
8. Load possible alternative routes.
9. Ensure database is the latest version.
10. Check the status on start-up.
11. Fly and navigate manually, only use the
GPS once you have verified its accuracy
against something else.
12. Do not rely on GPS for instrument
approaches.

[2Donkeys]

Let's leave aside for a moment, the question of whether a home-made approach is sensible. The two sides of that argument will never be reconciled.

The GASIL quote above contains generalisations about the way in which GPS receivers work. Many IFR-approved GPS receivers such as the Garmin 430/530 units do automatically improve their resolution as they approach their target, regardless of whether or not the target sits at the end of a GPS approach. See TERM and ENR modes in the user manual for details.

The RAIM comments are baffling. RAIM relates to whether adequate coverage will exist at a location at a specified time in order to be able to guarantee adequate 3D navigation. With the size of the GPS constellation of satellites, RAIM is becoming less of an issue generally - but it does not present a problem in the way described by GASIL.

Finally, the bland asssertion that GPS approaches are "not authorised" has no legal weight and no particular meaning. It is true that they are not "authorised" approaches, in the sense of their being officially approved. However, they are not illegal to perform as others have adequately explained earlier.

GASIL has produced a lot of this pseudo law-making since it underwent its change of editorial staff. In at least one instance, it has been required to publish a "clarification" in a following edition because of the errors contained in its original advice.

This is yet another example of a misleading article which should not be cited as authoritative, despite the organisation that publishes it.

2D

[VFE]

Interesting stuff folks.

Thanks.

VFE.

[High Wing Drifter]

The RAIM comments are baffling. RAIM relates to whether adequate coverage will exist at a location at a specified time in order to be able to guarantee adequate 3D navigation.
RAIM (Receiver Autonomous Integrity Monitoring) is more than just converge and is only loosely related to 3D fixes. For a 2D fix the RAIM process requires the interrogation tracking of three satellites. Every 20 secs (or so) one of the three is replaced with a fourth. If the position changes then the fourth satellite is rejected and another forth satellite is brought in. Therefore, in addition to the number required for the fix (3 for 2D, 4 for 3D) RAIM constantly required an extra comparison satellite (all verified).

The 2 miles off track error commented on by GASIL relates, I think, to the 20 sec latency in verifying a new satellite. If the erroneous satellite is at the back of the queue it could be 100 seconds before a position fault is identified as it cycles through the list and eventually realises that it has been tracking a dud.

[drauk]

HWD, interesting. If the 2nm error mentioned in the GASIL is a result of waiting for five 20 second cycles to pass how does the matter improve if the unit is in approach mode (assuming an IFR-approved GPS)?

[2Donkeys]

RAIM errors are produced when the almanac contained within the GPS predicts a loss of RAIM for the current position.

Predicted RAIM can also be checked for a given point at a given time on IFR approved GPSes.

Where signal loss occurs resulting in a loss of 3D nav (and a fallback to 2D nav), the GPS receiver will indicate a loss of 3D nav immediately. This too is a certification requirement for an IFR installation.

I am having trouble understanding the set of circumstances where an IFR-approved GPS will be 2 miles out without having considerably in advance warned of a loss of 3D nav and/or a RAIM failure.

Such an error would also apply to formally promulgated approaches dramatically limiting their use.... and no such limitation exists.

2Donkeys

[bookworm]

Perhaps it only happens to pilots whose baseline blood alcohol concentration approaches 20 mg/100ml even with total abstinence... ;)

[High Wing Drifter]

Drauk, 2Ds,

HWD, interesting. If the 2nm error mentioned in the GASIL is a result of waiting for five 20 second cycles to pass how does the matter improve if the unit is in approach mode (assuming an IFR-approved GPS)?
Good question. The technical answer is...I don't know :\

I can only imagine that the RAIM limits are increased so that a dud satellite will be rejected more readily and that all satellite must have been verified and that a minimum number of tackable satellites are present. Possibly in addition to ensuring that the cut of the satellite trajectories is nearer the optimum for the higher resolution CDI scale of 1 to 0.3nm in APR or terminal modes (shallow trajectories are worst). Further suppliment of psudeo satellite (psudeolites) ground stations and WAAS (Wide Area Augmentation System) availability (US).

WAAS provides very accurately known static positions of various ground stations that read their supposed position from the GPS netwok and correlate error values that are then broadcast indirectly back to the satellites to provide various corrections to increase the accuracy for mobile receiver units.

[2Donkeys]

Still no reasonable explanation of the 2 mile position error.

My original comment was that GASIL increasingly contains duff gen written from a specific angle. I stand by that and think that this is a good case in point.

I am yet to be convinced that there is a set of circumstances in which an IFR GPS will be 2 miles out without either providing a RAIM or "loss of 3D" warning.

WAAS has absolutely nothing to do with this. The US GPS approaches are not predicated on WAAS (although eventual precision approaches will be)


2D

[High Wing Drifter]

Agree WAAS is a bit of a tangent as it is US specific but many products are WAAS enabled specifically for Cat 1 GPS approaches so it is one aspect of how a GPS can make its approach more accurate.

Anyway, just found this http://www.asy.faa.gov/safety_products/GPSSafetyAdv.htm It specifies a RAIM warning limit of 30secs for ENR and 10 secs for APR with a future requirement of 6 secs for approach. If you are doing 240kts perpendicular to your intended track then yes, you maybe 2 miles out!!! Given this, I agree with 2D, it does seem a little far fetched.

[2Donkeys]

but many products are WAAS enabled specifically for Cat 1 GPS approaches

... none of which exist in a licensed form yet. I don't think anybody disputes the accuracy potentially available from WAAS... just that it is not relevant to the CAA's position.



On the subject of RAIM, it is worth remembering that those RAIM errors are based on the case in which the minimum number of satellites is available, and that no cross-checking can be performed on satellites being receiving on other channels.

In reality, IFR GPSes are specified to receive on a higher number of channels, so that an odd satellite can be identified much faster. Satellite coverage of the UK and US is now much better than it was when GPS and RAIM were first envisaged reducing this scenario's relevance to below that associated with signal distortion on NDBs and the like.

Anybody ever heard of Night Effect, Coastal Effect etc.. :D


2D

[IO540]

My "favourite" NDB/DME proc is rarely less than 30 degrees out on the inbound. Everybody on the airfield knows this. I don't understand how anybody can defend this with a straight face.

You mention WAAS is not relevant to the CAA position. Is anything to do with GPS relevant to the CAA position? (This is a serious question)

[2Donkeys]

You mention WAAS is not relevant to the CAA position. Is anything to do with GPS relevant to the CAA position? (This is a serious question)

Relevant in this context meant, relevant to the position as stated in the above GASIL quote. [Incidentally, that quote also features in the current CAA GPS Safety Sense Leaflet]. WAAS is not relevant to any discussion of the CAA's position in that document.

More generally, it is my understanding that the CAA maintain the position that they are happy to consider applications by airfields for a GPS approach - but that they have yet to receive such an application.

You may be aware that the relevant ICAO Annex lays down standards for such approaches. The CAA would presumably take those standards as the basis for validating any such approach.

GPS approach trials were flown at Cranfield extensively a few years ago (by the University I believe), and I am also led to believe that the RAF have an "approved" GPS approach defined for Kemble - not relevant for civil ops, but interesting nonetheless.

2D

[Keef]

I have a strong suspicion I flew an approved GPS approach in California (albeit on a glorious sunny day) and found it a) extremely accurate and b) no big deal. Certainly a lot less hairy than an NDB approach at night into a certain Florida Airport with an Italian-sounding name.

Give it time. GPS approaches will come to Europe (once the Galileo satellites are up and EASA's found a way to charge us for using it all, maybe).