TCAS Philosophies

TCAS philosophies I decided to start this thread as a follow up of the “ Ueberlingen trial “ thread which was used by some to re-open what I call the “ TCAS Philosophies “ If a mod would like to move the posts in the Ueberlingen trial thread here, welcome, but not necessary. The TCAS system is very complex, there is no more than a dozen persons in Europe and in the USA who fully grasp everything, and no more than a few more dozens that understand more than 50% of it . ( I classify myself in the second category ) What is below is not a lecture, this is basic stuff to help you understand where we are . A few facts : The Ueberlingen collision revealed to the public ( but not to the specialists) a few flaws in its logic , mainly that the RA sense reversal did not work. (That is when one aircraft follow the RA , but the second maneuvers in the opposite direction of its RAs and both aircraft end up towards each other ). There was request made from Eurocontrol to re-open the work on TCAS and make a software change to correct this flaw, but the FAA refused initially as version 7 was “ final” in their eyes and that the TCAS Team had been disbanded. Nevertheless though RTCA , discussion were restarted and what became known as Change Proposal 112 ( or CP112 ) became a fact, later it became 112E ( enhanced) , and , since we were re-opening the Pandora box, we added a few more flaws corrections mainly one ( CP115) on the replacement of the RA ‘ Adjust vertical Speed ,Adjust “ to :” Level off , Level off “ and another one (CP116) on weakening RAs at low altitudes . There is a chance now that we will get a new version 7.1 in the” future” . I do not put “ near future ” as the date is still fought between Eurocontrol which would like 2010 for all, and the FAA which says “ much later” for CP 112E ( sense reversal ) and is against CP 115 , the “Adjust/ level off “ change . In case you are lost in the TCAS / ACAS versions here is a short recap : TCAS is a brand name. ICAO call its Specs ACAS 2 current sofware versions of TCAS flying around : 7 and 6.04 A Udate to 6.04A was free, 7 was not . 7 was pushed by the Europeans, FAA was against it. 7 is RVSM compliant , 6.04A is not. If a new version comes ( 7.1 ) , will it be mandated ( i.e free) or will it be recommended ( at a cost ) is a good question to which I do not have the answer. Initially there were 3 TCAS intended , TCAS I : TAs only ,

TCAS II RAS in vertical plane, TCAS III RAS in both vertical and horizontal plane. Work on TCAS III stopped as the logic proved too complex, and vertical plane RAs were more effective at avoiding a collision. So there will most probably never be any TCAS III . This is the situation today . Now questions asked were : is a new Ueberlingen possible today : With TCAS sense reversal logic flaws : yes , nothing has changed since 2002. same logic. Same version flying around (7.0). With procedures : maybe : ICAO has revised its documentation and most training material have highlighted the way to follow in case contradiction between ATC clearance and RA. But incident statistics so far both from USA and Eurocontrol show that some controllers still issue clearances contrary to RAs and that pilots still choose to follow ATC instead of RA. ( a recent case in France, with US trained and experienced 767 pilots from a major US airline , so no need to think it only affect exotic or Russian pilots ) Main reason : lack of understanding on how humans and automated machines function in reality. Prof Ladkin in his excellent paper http://www.rvs.unibielefeld.de/publ...rts/SCSS04.pdfon causal analysis after Ueberlingen call this “ Sociotechnical systems “ . Now a last statistic : ( coming from Bill Thedford, of the Boston MIT LLC, one of the top 12 in my first classification earlier) Ueberlingen type encounters ( sense reversal ) occurs at 4.7 x 10 to the minus 6 per flying hour , or 58 events a year in Europe .If we expand this model, Bill predict that we could have 1 Ueberlingen –type mid air collision every 4 years due to the logic flaws. The conclusion of this all : Do not philosophy about TCAS : It is too complex a system. My advice : if you are a pilot : follow the RA, and if you are a controller and you have to give anti collision instructions , give a turn as well, in case TCAS comes in , it will complement your instruction instead of nullify it. If you found this too long , this was the ( very) short version. Safe flying.

11th September 2007, 10:23

PBL Join Date: Sep 2000 Location: Bielefeld,

#8 (permalink) Quote:

Germany Posts: 559

T Originally Posted by airmen, referring to TCAS they saved already my live once at least! here is a phenomenon here worth thinking about. When I voiced some of my concerns about TCAS on a pilot's mailing list some years ago, I was deluged with affidavits from pilots who claimed that TCAS had saved their lives. If all of these claims had been true, that would have amounted to some 20 or so midair collisions in a sample of a few hundred pilots (the members of the mailing list) over the course of, say, ten years. Let's assume 10 "life-savers", that is, otherwise-midairs, for 500 pilots in 10 years. That is an average of one "midair collision" per 500 pilots per year. There are about 15,000 aircraft from Airbus, Boeing, MD, Lockheed still in service (calculated from figures in Flight International, 24-30 October 2006). So that's not counting the "commuters", or the Tupolevs. Those aircraft are in service, let's say, 14 hours per day, for a total of 210,000 hours per day, or 5,880,000 hours in 28 days. Duty rosters take 100 flight hours for a pilot in 28 days to be relatively high, but let's use this figure, since it will lead to a conservative estimate. It means there are about 59,000 captains needed to fly those 5,900,000 hours every 28 days. With one "midair" per 500 captains per year, and accounting for the fact that it takes 2 captains to have a "midair", we come to a figure of 59 "midairs" per year. Compare this with actual midairs amongst airline transport aircraft. 1959 Grand Canyon, 1976 Zagreb, 1978 San Diego, 1985 Cerritos. And then, since the advent of TCAS, 1995 Namibia (involving a German military machine which I do not believe was TCASequipped), 1996 New Delhi, 2002 Überlingen and 2006 Amazonas. That is 4 before the advent of TCAS (mandated 1991, I believe) and 4 since (16 years). Those crediting TCAS with all these "saves" need to explain why, before the advent of TCAS, there were a statistically-negligible number of collisions in the history of airline flying, and upon the introduction of TCAS there are suddenly almost 60 per year. Even considering the growth of air travel, that is an unbelievable jump. The obvious answer is, of course, that most of those "saves" would not have resulted in collisions, despite what their pilots thought or think. Consider the following, as a thought experiment. Up to the Congressional mandate for TCAS, there were

three collisions involving airliners in the U.S.: 1959 Grand Canyon, 1978 San Diego and 1985 Cerritos. Suppose TCAS initiated RAs, not for 2 TCAS-equipped aircraft, but only when one had TCAS and the other only Mode C. That would have satisfied the issue worrying the Congresspeople, namely incursions of GA aircraft into zones of intensive airline operations, and it would have saved one of the midairs since (Überlingen). The other three midairs in the TCAS era occurred to non-TCAS-using aircraft. Let us compare TCAS with another highly-lauded system for avoiding accidents, GPWS/EGPWS. Also a system introduced primarily by one manufacturer (again Honeywell). And in contrast to TCAS addressing a real, continuing problem which still leads to many aircraft losses per year, namely CFIT. EGPWS is essentially a private development (stemming from the work of Don Bateman and colleagues). It is interesting in this regard to notice how TCAS is self-advertising and EGPWS not. A pilot can say "TCAS saved me" without it reflecting on himher self, but rather on some air traffic controller establishment somewhere. No pilot can say "EGPWS saved me" without someone else asking how heshe lost situational awareness in the first place (with rare exceptions in which it is clear how, such as with British Mediterranean at Addis Ababa). So the statistics were never there to say "TCAS is a good thing", and it is self-advertising, through pilot "save" stories. But there are smart people in aerospace who, no matter whether they think TCAS is a net gain or a net problem, are able to look at the facts and the situation just the same as I am now. Airline CEOs, for example. So the question is: why are collision avoidance mechanisms still being promoted, researched and supported to such a great extent? I think there are two reasons. One is that they are one attempt at a solution to a problem which no one knows yet how to assess accurately, namely the avoidance of midair collisions, and no one is willing to risk misjudging (underestimating) the likelihood of those. The other reason is that effective on-board systems make collision avoidance a contract between the two (or more) aircraft involved, which transfers the costs of collision avoidance away from large ATC organisations. Since the airlines and their passengers ultimately fund these organisations, one could see the financial benefits to airlines of bringing this function in-house (after the initial cost of installing the equipment, which has n any case been

mandated). Add to this that collision avoidance is one of the brakes on moving to "free flight" (self-routing IFR at high altitudes), which airlines believe would bring them great cost reductions, and one can see that TCAS+free flight is a politically sellable package which could reduce the costs of air travel even further. Without some ATC-independent collisionavoidance system regarded as effective, this move could not happen, and TCAS is the only technology in town. PBL

airmen Join Date: Nov 2002 Location: planet hearth Posts: 110

TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders? Maybe those guys need to have a good Sim session to understand by doing it wrong to be able to learn something? PBL wrote:

Quote:

I The obvious answer is, of course, that most of those "saves" would not have resulted in collisions, despite what their pilots thought or think. saw the conflicting aircraft (a Beech Baron) at the last minute (sun in the back) during the escape manoeuver, he was very close and was flying opposite course at the same altitude (controller error) and I can tell you that the controller went by himself to meet us after landing, he told us that he saw nothing and as such gave us no instructions. Off course we followed the TCAS but I can not tell you what the other pilot did...

Last edited by airmen : 12th September 2007 at 07:44.

12th September 2007, 10:05

ATC Watcher Pegase Driver Join Date: May 1997 Location: Europe Age: 59 Posts: 1,052

#19 (permalink) The " value of X " Quote:

I One of the main issues with TCAS, for me, is that no one knows what the value of X is. ndeed , and that debate is hindering the obvious solution to our problems : making the RA a command, i.e. an obligation to follow in all circumstances. In the US, at its conception ,the MITRE/FAA Lawyers prevented this and insisted we call the TCAS avoidance resolutions ' Advisories " and not " Commands" therefore it is called an RA. But besides this " value of X " technical issue , there are other aspects as well. I always refer to the " German glider " one , following a presentation by a famous German 747 Capt in ICAO : to explain why he will never agree that following RA a should be re mandatory in all cases he said : ,I am in IFR in class G airspace and I get a climb RA , but just above me are gliders without transponders , my first duty as Captain is to maintain the safe operation of my aircraft and stay below the gliders , therefore I will elect not to follow the RA in this case. And he is right. So an advisory it remains.

Quote: I TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders? ndeed why argue ? Mainly for 2 reasons : one is human : people react differently to automation that others and culture also plays a role. PPRuNe is full of discussions on this issue . Studies shows that even as we speak today 30 to 40% of the " Adjust vertical speed " RAs are not followed by pilots. ( it used to be

60% 3 years ago ) . So it is not only ATC " interference". The other reason is the risk , the " value of X" mentioned above. If it would have been all without risks, , coupling the RAs to the auto-pilots would have resolved the issue and would have been done.

12th September 2007, 10:06

PBL Join Date: Sep 2000 Location: Bielefeld, Germany Posts: 559

#20 (permalink) Quote: Pl Originally Posted by airmen TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders? ease read my analysis of the decision problem presented to the Bakshirian crew at Überlingen. They were faced with an "intruder" at 10 o'clock which they saw, and an unknown conflict at 2 o'clock which they didn't see, and for which they had an advisory to descend (that is, he was at or above their altitude). What would you do? Climb towards another conflicting aircraft that you don't see but ATC does? Or avoid him, descend towards an aircraft that you do see, and hope to avoid him using visual means? Please give some good reasons for your answer that will also be good reasons for any other pilot in this situation.

Quote: That's very interesting and I agree with you that it counts as a save (but of course it doesn't contradict my observations on claimed "saves"). I have a number of observations. First, Beech Barons don't have TCAS (I don't know that the avionics are approved for installation in a Baron,

does someone?). So he is painting Mode C, otherwise you wouldn't have picked him up on your TCAS. If he is painting Mode C and you picked him up, then how come he is not showing up as the usual box as big as yours on the controller's screen? I think there is a lot more to this incident than you are able to tell us. If the controller said he saw nothing, and he was telling the truth, then it must have been a system failure rather than controller error, and his going to meet you and apologising was someone else's way of covering up a technical system failure. PBL Very interesting. The difference between the "German glider" decision problem and the Bakshirian one is that the B747 knows the non-painted conflicts are above him. Bakshirian doesn't know that. The "German glider" decider is therefore able to maintain altitude without conflicting with non-painted traffic, and equally able to hope that the intruder is TCAS-equipped and responds to a descent RA. The "Bakshirian" decider is not able to maintain altitude without conflict with non-painted traffic, as far as he knows. The German-glider decider can follow the dictum not to manoeuvre contrary to an RA. The Bakshirian decider cannot follow this dictum without risk. PBL

12th September 2007, 10:17

#22 (permalink)

Capt Pit Bull

Peter,

Join Date: Aug 1999 Location: England Posts: 667

Quote: That might be stretching it a bit. You can have EGPWS warnings when the aircraft is physically in the correct part of the sky. A good example is a that map shift on final approach can easily lead to an escape manoeuvre. We had a spate of this on one type I flew. Now, flying a go around from that point in the sky shouldn't (in theory) unduly tax ATC but at a busy multi runway international airport..... well, you get the picture.

Not that I disagree with the broad thrust of what you are saying. pb

17th September 2007, 21:41

alf5071h Join Date: Jul 2003 Location: An Island Province Posts: 649

#37 (permalink) Peter, the ‘GPWS’ situation is more complex if you consider EGPWS which uses a database and navigation position in its computation. You may have meant this but your response related to GPWS; as you realise, it is important to distinguish between the two. This is particularly so for the crew as the later system (EGPWS/TAWS) is more capable and reliable than previous systems, but then opens greater opportunity for error due to the range of alerting and warning modes and the terrain display, e.g. during an approach a pilot may pull up in response to a warning and incorrectly level at MDA, but due to a (gross) error in range, MDA is not safe. Also note that some versions of EGPWS use GPS altitude as a gross error check of altitude, mainly incorrect pressure setting. GPS (geometric) altitude does not replace the rad alt (there are exceptions) as its accuracy is less than that of normal altimetry, and therefore at this time it would not be a candidate for ACAS altitude as suggested earlier in the thread. As for the solution to the three aircraft ACAS problem, this is done reasonably successfully by combat pilots in 1 vs 2 situations. My experiences and existence today is relevant, but based on the same experiences I would be less confident for situations involving 4 vs 4, due to the human limitations in continuously computing a 4D (space – time) solution, that’s assuming that you can see everyone! Your ACAS problem relates to a similar 4D situation and thus might be modelled in the form the 4th power. Considering three aircraft avoidance, x, y and z, then a solution could be in the form of x^4 = y^4 + z^4. This form of equation for powers greater than 2 has been proven to have no solution (Fermat’s conjecture). With appropriate deductive logic, ‘no solution’ relates to a collision condition, thus I claim

my PhD !

17th September 2007, 22:01

ATC Watcher

#38 (permalink) Quote: I

Pegase Driver Join Date: May 1997 Location: Europe Age: 59 Posts: 1,052

If this thread was about a crew's response to a GPWS alert: "Whoop Whoop PULL UP!", which involved pushing the stick forward instead of back, would there be much in the way of technical discussion required? nteresting point .Because , if we go back in time, there are similarities indeed . If one remembers the introduction of GPWS, the early systems were prone to many false alarms, which worked against crew acceptation. ( remember for instance the Air Inter decision in France with their new A320 fleet, and the discussions that followed after their Mt St Odile CFIT ) I do not have precise archive data at hand , but I remember there were quite a few CFIT accidents in the late 80’s /early 90s , where GPWS alerts were ignored by pilots. ( the Independent air Boeing 707 in Santa Maria /Azores in 1989 , or the Thai International A310 in Katmandu in 92 are just two that come to my mind for instance ) So the “ philosophies ‘ ( how to deal with automation ) do apply for both systems, in their early phases. In 2007, GPWS , and especially EGPWS could be considered mature. Not TCAS. TCAS is, despite its age, still in its early phase , as we now on version 7.0 of its development, and that after a collision and a good dozen of very,very near calls , we are most probably going to have a version 7.1 in a few years. That might resolve partially the flaws that contributed to those accidents/incidents. What we do in the meantime( i.e. until that new version comes ) , is part of the debate here.

18th September 2007, 00:14

#39 (permalink)

FullWings

PBL,

Join Date: Dec 2003 Location: Tring, UK Posts: 432

Quote: I agree with 1. to 3. but would you agree that 4. is likely to happen near an airport (you have to approach the ground when landing and get near it when taking off) and airports are well known as places where aeroplanes like to congregate... Anyway,

the points I'm trying to make are: a) GPWS and TCAS (and to a lesser extent EGPWS) are last-defence systems, conceived to possibly stop/reduce the chances of an accident occurring. They are not designed or certified to replace ATC or good piloting - except at the last minute when either or both have been lacking. so, b) As the alerts/advisories/warnings given by the above equipment are somewhat time constrained in their period of usefulness (!), then a consistent response over a short period is required to allow these systems to function as intended, i.e. you can't pontificate at great length about what action to take: it must be a memory drill - "recall item". And you have to perform it correctly.

Quote: I assume that is the Überlingen scenario? I say that it was the worst possible because it led to a collision; almost by definition any other action would have produced a "miss". I know this is slightly simplistic but if you think about it in reverse, to generate a "hit" both aircraft have to occupy the same small space over the same very small time period. Any changes to the trajectories of either will quickly disrupt this meeting. Finally, it's because the manual says: "NEVER MANOUVER AGAINST AN RA".

Quote: F Those are not the only two options. Another arises when you have an advisory to manoeuvre to avoid a conflict from same-level or higher, and you have an aircraft below you in sight. What would your decision be and why? ollow the RA. The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of. I would posit that you are unlikely to hit an aircraft that you have in plain sight as you can tell if it is on a constant relative bearing or not; also you have the option of a lateral manoeuvre to de-conflict. If the a/c below has TCAS, there might be coordination going on that you are unaware of. If it doesn't, then they'll (like you) probably follow the rules of the air to avoid a collision (if they've seen you!) I find detailed technical discussions about most things to do with aviation absolutely fascinating... but the job has shown me that for some scenarios, you have to have a fairly rote response prepared for immediate use. I applaud scientific examination of the limits of

aircraft systems but when you get to 99.99% confidence (or whatever) in a particular one, especially if it involves time-critical warnings, you just have to say: "It works, do what it says" as there isn't any room left to do a risk analysis on an alternative response. I have had several RA's in real life, one caused by an aircraft directly above deciding to descend at a great rate through our level. Did TCAS save our lives? Don't know but it was f***ing close when we did eventually see it.

18th September 2007, 07:39

#40 (permalink)

mono

Dynamite,

Join Date: Feb 2002 Location: UK Posts: 396

I will try to answer a couple of your questions. EPR as you correctly state is a measure of the thrust produced by a Jet engine. It is, as its name implies, the ratio of what comes out the back over what comes in the front. The inlet pressure P1 is sensed by a single probe (like a pitot probe), which is usually inside the engine inlet cowl, but can be on the spinner (B727) or the pylon (B707). The exhaust pressure P7 is sensed by a manifold or rack of tubes with holes in the jet pipe. In the main all EPR indicating systems will, assuming TO power is acheived before the a/c is rolling, show a decrease in EPR as the a/c accelerates down the runway. and this is quite simply because the pressure at the inlet probe increases as the ram air effect is sensed at the probe. A/c with the probe mounted in the inlet will only show a small decrease because the probe also senses the pressure of the air being sucked into the engine. The old 707 however, with its probe mounted on the pylon, away from the air being sucked into the engine used to show a marked reduction in EPR as it trundled down the runway. Cost index is a figure provided for the a/c usually by the airline planning and performance dept. I can't give you any specifics, but it is used by the FMC to compute optimum climb and cruise performance levels. A low cost index will cause the FMC to calculate performance levels to provide a lower fuel burn. With a higher cost index, fuel burn becomes less important and speedy enroute times more important. The final figure is a compromise between fuel burn and enroute time and may change depending on the route being flown. INS, ahh, that old chestnut. The first and most important thing to realise is that neither INS or IRS use

gyros to sense north. The gyros are merely there to either electronically or mechanically maintain the INS/IRS platform level and aligned with true north. The system senses north in the following way:- there are 3 sensitive acceleromerers each aligned 90 degrees relative to each other. We'll call them the vertical, lateral and horizontal accelerometers. During the initial alignment process, the outputs are used to calculate the vertical axis (in the old INS systems the platform was actually moved by motors so that the vertical accelerometer was physically aligned with the vertical). Once this is known then the outputs from the accelerometers are used to sense acceleration forces due to the rotation of the earth and it is the resolution of these forces that aligns the INS along the true north/south axis. Note that due to the obvious equitorial ambiguity (acceleration forces at 40 degs N and 40 degs S are the same) it is not until the a/c present position is put into the INS that true north is known. Hope this helps

17th January 2003, 23:11

Notso Fantastic Join Date: Aug 2000 Location: UK Posts: 1,814

#3 (permalink) Interesting questions! Maybe the EPR does change. Never been looking at it to notice. On the Boeings I've flown, autothrottle sets the required thrust and i haven't been aware of it modulating the thrust levers to maintain constant thrust. I think any change on big fan engines must be very minor. Question 5- back in the 70s I was dragged out on standby to copilot a Certificate of Airworthiness test on a VC10 fitted with an AoA meter. Following the incidents of superstalls on Tridents and BAC 1-11s (always fatal), I was not altogether happy to find myself doing stick pushes at 15,000' over Anglia. Speed was reduced to below 100kts and AoA hovered at 15 degrees, then twitched, then jumped to 17 degrees. The stick push horns cut in and the stick was pushed forward and suddenly there was a nice view of farmland. Nice to be alive and enjoy it, but Notso couldn't help thinking 'very good theory, but what if it doesn't work?- I'd really rather be home reading the Sundays!' Back to your question- I assume if you stalled in a bank situation (increased 'g'), I think the AoA would be different? To expand slightly on INS & True North, systems cannot sense the direction too near the Poles, so INS sets, although they can fly over the Poles, cannot be aligned preflight near the Poles. I forget the limitations, but I think if a 747 started out within

about 20 degrees latitude from the Poles, the INS sets would have difficulty aligning themselves. Used to take Classic 747s about 20 minutes, the 400 version about 7.

2nd December 2002, 17:01

None Join Date: Jul 2000 Location: West Posts: 345

#8 (permalink) I am going to training this week, so I thought I would look into this question. Below is from the US Aeronautical Information Manual (AIM) Chapter 4. 4-4-11. SPEED ADJUSTMENTS a. ATC will issue speed adjustments to pilots of radar-controlled aircraft to achieve or maintain required or desire spacing. b. ATC will express all speed adjustments in terms of knots based on indicated airspeed (IAS) in 10 knot increments except that at or above FL 240 speeds may be expressed in terms of Mach numbers in 0.01 increments. The use of Mach numbers is restricted to turbojet aircraft with Mach meters. c. Pilots complying with speed adjustments are expected to maintain a speed within plus or minus 10 knots or 0.02 Mach number of the specified speed. I'm uncertain of its applicability outside of the USA.

29th August 2002, 17:11

cwatters Join Date: Dec 2001 Location: England Posts: 927

#3 (permalink) Wings with a lot of sweep need less dihedral because the sweep contributes to roll stability (don't as me how, I don't know). If you have too much roll stability the plane can't turn fast so they reduce stability to a satisfactory level by giving swept wings anhedral. Paragliders have anhedral for a different reason - to help keep the wing open I believe.

29th August 2002, 18:08

erikv

#4 (permalink) cwatters, Swept wings improve directional stability.

Join Date: May 2001 Location: Netherlands Posts: 111

For example, yawing to the right causes the left wing to be positioned more perpendicular to the incoming airflow. This in turn increases the drag of the left wing, causing the a/c to yaw back to the left. Erik.

29th August 2002, 19:39

#5 (permalink)

Keith.Williams.

Parapunter

Join Date: Aug 2001 Location: Dorset Posts: 597

The above replies are correct, but overlook the specific problems facing the harrier designers. In order to produce efficient vertical thrust, the gas flowing from the engine nozzles requires a reasonably clear path vertically downwards. This required the use of high wings. But high wings combined with a fairly high wing sweep angle would produce unacceptably strong lateral stability. The addition of anhedral both on the wings and tailplane, reduced the larteral stability, thereby restoring a reasonable degree of roll response.

29th August 2002, 22:04

#6 (permalink)

John Farley

Keith

Do a Hover - it avoids G

I have grave reservations about sticking my nose in on this one, but for the first time ever I could find myself at odds with something you have posted.

Join Date: Oct 1999 Location: Chichester West Sussex UK Age: 76 Posts: 1,099

Your last sentence was “The addition of anhedral both on the wings and tailplane, reduced the lateral stability, thereby restoring a reasonable degree of roll response.” I can only agree with this if the roll response you quote is that due to a lateral gust. If it is the roll response due to aileron deflection that you mean, then I disagree. The rate of roll that happens following aileron deflection on any aircraft has much more to do with the roll damping of its wing rather than any lateral stability it may possess. Static lateral stability of the sort produced by dihedral ONLY produces a stable righting moment (that opposes ailerons) if sideslip also develops from the side of the down going wing. It is the need to obtain a reasonable (as opposed to huge) response to a lateral gust that leads people to

employ anhedral. Without it an aircraft with a high wing and low CG, especially if the wing is also swept, will have a terrible (huge) tendency to roll when hit by a lateral gust. So whether it is a 146 (where you are concerned about passenger comfort as well as controllability on a gusty approach) or a Harrier (where aiming steadiness and low level ride are the considerations) the answer is to include a rolling moment due to sideslip that has the opposite arithmetic sign to that coming from sweep plus high wing and low CG. This means anhedral. You can even use huge lateral stabilty to generate high roll rates. Nearly 40 years ago as a very new tp just posted to Aero Flight I was given the SB5 (Lightning look alike with a tiny donk and a fixed gear) to fly on an open day. All of the experienced guys having declined to be seen dead in the device picked rather more manoeuvrable mounts (like the FD2 or the HP115) On my first flight I found why: the stick forces were huge, the rate of roll from aileron was negligible (thanks to them being unpowered, very close to the fuselage and being almost too heavy to deflect). But the monster had a sweep of 60deg so this huge lateral stability enabled a modest facsimile of a twinkle roll to be produced by kicking the rudder and making deliberate use of the huge rolling moment due to sideslip. Regards John

30th August 2002, 12:45

Mark 1 Join Date: Aug 2000 Location: Warwicks, UK Posts: 652

#7 (permalink) As far as Rolls-Royce go, this seems a very confused subject. From what I understand the RB series were originated in Bristol, although their is no RD series for

Derby. The 211 was just a sequential model number, most of which never saw the light of day. The -524 followed from the -22, I think the 5 indicated it was 50,000 lb thrust class, although the -535 was only (originally) 35000lb class. The other letters referred to throttlepush or growth variants of the same engine. As to why the RB211 didn't get named after a major river (as in Spey, Dart, Tyne, Trent etc.), I've never heard an explanation.

4th August 2002, 19:07

#4 (permalink)

Babi Melayu

Please don't call yourself a retard.

Join Date: Feb 2001 Location: Malaysia

You might be familiar with the more common 3-degree

Posts: 13

glideslope or gradient of 5%. The 5% means 5 / 100, 5 feet lost vertically (y-axis) for every 100 feet travelled horizontally (x-axis). When you intend to find the angle subtended by this 5 (y-axis) and 100 (x-axis), you just take the inverse tangent of them both - opposite over adjacent i.e. inverse tangent of 5 / 100. The result is an angle of 2.83 degrees ( which is close to the 3-degree we mentioned). The TWELVE percent (12%) slope means you lose 12 feet vertically for every 100 feet travelled horizontally and this is simply a much steeper slope i.e. a whopping 6.84 degrees glideslope! ( Inverse tangent of 12 over 100 gives us 6.84) Who uses the 12% slope, by the way?

Last edited by Babi Melayu : 4th August 2002 at 19:14.

9th August 2002, 11:37

BlueEagle Join Date: May 2002 Location: Australia Posts: 2,252

#5 (permalink) On a clear day a visual approach to R/W13 was possible and the CX pilots often did it, they being very familiar with the place. For the rest of us it was usual to do an IGS, (Instrument Guidance System), approach which gave you Localiser and GP information down to a height of approx. 650' but on a track 45degrees to the R/W QDM, by which time you should be visual with the runway and able to complete the turn on and landing visually.

11th August 2002, 21:11

#7 (permalink)

ShyTorque Avoid imitations

SuperTed,

Join Date: Nov 2000 Location: Still wandering in hyperspace. Posts: 5,089

It has got nothng to do with static pressure or the capsule. A vibrator in an altimeter is there to

overcome stiction / friction within the mechanism, i.e. cogs and a suitable system of levers and pulleys etc between the capsule and the indicating needles. Without it an altimeter needle may lag / jump and operate in "steps". A vibrator just smooths out the movement of the needle. Note that it isn't often necessary to fit one to helicopters or piston engine aircraft that vibrate through other means.

14th January 2002, 12:06

#7 (permalink)

Shore Guy Join Date: Jul 2000 Location: U.S.A. Posts: 407

In one of my first (in person) introductions to the great dry Britsh wit, I was in Sim 1, Day 1 on the BA-146 at Hatfield. Instructor in the back, using his pointer to go over all the instruments, switches, indicators (retired BA fellow as I recall). He pointed at a red flag in the standby attitude indicator, and

said "It's time to get out the flight attendants emergency checklist - it seems our vibrator is inoperative".

8th February 2001, 12:00

#5 (permalink)

BOING

Posts: n/a

If you want a quick mental conversion that works quite well at lower altitudes. TAS = IAS + (IAS/60 x Ht (in thousands of feet)) IE TAS = 240 + (240/60 x 10) = 280 Try it and see if the answers are close enough for your use. ------------------

9th February 2001, 05:16

#7 (permalink)

TOGA_Party

Posts: n/a

Smurfjet, Not so much a formula as a 'rule of thumb'. Works when conditions are close to ISA and at altitudes not flight levels. The TAS and IAS will vary by 1.8 kts/1000'. eg. If we're cruising at 10'000 and indicating 145kts then TAS will be 163kts. ie 1.8(kts/1000')X 10(lots of 1000')= 18kts 145kts (IAS) + 18kts = 163kts (TAS) Also works just as well the other way around!!

10th February 2001, 18:08

#8 (permalink)

fart

Posts: n/a

This will give you a quick answer that comes faily close to the real deal:

Use 2% of IAS per 1000 ft and add to IAS: Example: IAS is 200 Knots at 25 000 ft therefore 2% X 25000 divide by 1000 = 25 = 50% of IAS 50% of 200Knots + 200 KNOTS = 100 + 200 = 300 Knots TAS

22nd February 2001, 15:22

#3 (permalink)

Tinstaafl

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24th February 2001, 05:51

I know it's a bit pedantic but I know that rule of thumb as TAS/10 + 7, not IAS...

#7 (permalink)

Dan Winterland

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Each PAPI light is like a slide projector with a red bottom segment white top segment. The four lights are set at a different angle, so that when you are on the correct glideslope you see two reds and two whites. The slope set corresponds with the ILS glideslope angle and will lead you to the radar touchdown point, usually 1000' in. If it isn't an ILS runway, the PAPIs are set at 3 degrees. When you are on the slope, the red PAPIs are next to the runway. That may not seem important, unless you are a CAA examiner. I was asked that in my air law exam, and couldn't remember despite having stared at them regularly for over ten years!

11th February 2000, 02:59

#3 (permalink)

shakespeare

Posts: n/a

The thing to remember with PAPI lights is that they bring you to the same touchdown point. i.e. 2 whites and 2 reds = 3 degree glide path. 3 reds and 1 white (flown constantly) reduces the angle but you will touch down at the same point. The same applies flying 3 whites and 1 red, however with a higher angle.

Hope that helps!

29th January 2001, 08:06

#8 (permalink)

Yoeman

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Precision Approach Path Indicator (PAPI) was invented in England .. the name Tony Smith is often associated with this invention .. which is probably true .. PAPI was adopted by the International Civil Aviation Organization (ICAO) and was to replace the older Visual Approach Slope Indicator (VASI) by 1995 ... this was based on the premise that the life of VASI was 15 years, so that when it came time to replace VASI, it could be done with PAPI ... however, the reality was that some countries did not have funding arranged to do this, so VASI continues at many sites ... PAPI is the same as VASI in the sense of the light units ... all that has been done is to relocate the units to the "theoretical touchdown point" and to vertically align the four units A, B, C, and D separately at about 20 minute increments of A=2'30", B=2'50", C=3'10" and D=3'30" ... the units B and C define the "approach corridor" with which the approach path (slope) is (B+C)/2=3'00" ... pilots fly the "approach corridor" (two whites and two reds) and not the approach path since the path itself is not seen ... the PAPI signal is not coincident with the ILS path ... it can only be made to be approximately close to the ILS path through increasing the width of the approach corridor which is indicated by the color signals ... the PAPI is a visual aid information on PAPI is available in the ICAO Annex 14, FAA advisory circulars and Transport Canada manual TP312 ... as well as in the pilot flight manuals ...