AF 447 Search to resume (part2)
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Everyone has been looking at them long enough, but here is what they sound like.
http://www.astrosurf.com/luxorion/Radio/mode-acars.wav
http://members.optusnet.com.au/extre...unds/acars.mp3
http://www.astrosurf.com/luxorion/Radio/mode-acars.wav
http://members.optusnet.com.au/extre...unds/acars.mp3
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Graybeard,
There is a "feature" of HF that renders your comment pretty much meaningless, at least for reception.
The aircraft would be operating well below 10MHz and probably around 5MHz or even 3MHz at that hour of the night in June of 2008. Above those frequencies the propagation would simply not be there. At those frequencies you can use a receive antenna that is only 10% efficient or even less and still receive as well as a "good" antenna would receive. Ambient noise that cannot be escaped makes up the difference. At 3MHz it's on the rough order of 20dB above "Boltzman" or theoretical minimum noise levels. (When on those bands I always intentionally put in 10dB to 20dB of attenuator following the antenna. It makes the receiver's so called "intermodulation distortion" (birdy level) lower.) This noise is an issue until you get up around 100MHz give or take a little.
The aircraft would HEAR DAKAR just fine. And unless DAKAR had storm static to worry about they'd be able to hear the plane as well as they ever hear the plane.
There is a "feature" of HF that renders your comment pretty much meaningless, at least for reception.
The aircraft would be operating well below 10MHz and probably around 5MHz or even 3MHz at that hour of the night in June of 2008. Above those frequencies the propagation would simply not be there. At those frequencies you can use a receive antenna that is only 10% efficient or even less and still receive as well as a "good" antenna would receive. Ambient noise that cannot be escaped makes up the difference. At 3MHz it's on the rough order of 20dB above "Boltzman" or theoretical minimum noise levels. (When on those bands I always intentionally put in 10dB to 20dB of attenuator following the antenna. It makes the receiver's so called "intermodulation distortion" (birdy level) lower.) This noise is an issue until you get up around 100MHz give or take a little.
The aircraft would HEAR DAKAR just fine. And unless DAKAR had storm static to worry about they'd be able to hear the plane as well as they ever hear the plane.
@Pugachev_Cobra:
Thanks for posting the original French text. Should have looked it up myself earlier. From what is left of my French I see that it is a bit different from what I read into the English description.
The French text does not point to a re-activation of the AP. It is merely stating that if the AP was connected before, it would have been disconnected as a consequence of this message.
So I take it, I read to much into this message.
Conclusion for me: Attempted/temporary re-activation of AP still possible but no direct hint towards it, at least not from this message (I hope they got at least the French text right...)
Thanks for posting the original French text. Should have looked it up myself earlier. From what is left of my French I see that it is a bit different from what I read into the English description.
The French text does not point to a re-activation of the AP. It is merely stating that if the AP was connected before, it would have been disconnected as a consequence of this message.
So I take it, I read to much into this message.
Conclusion for me: Attempted/temporary re-activation of AP still possible but no direct hint towards it, at least not from this message (I hope they got at least the French text right...)
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Is the AP going back automaticly if the a/c "think" the speed indications are right back?
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ACLS65, thanks for the link.
Just having been reading through most of the English version of the 1st June 2009 report.
Some of it is clearly translated French, but none looks bad enough to lead to obvious misinterpretation by people familiar with the subject.
mm43 (re your earlier post), I would call it acceptable, but since I know French I can see where some of the terminology would have come from, even without having the French original next to the translation....
Whether it's as understandable to somebody without any French... I can't really judge.
HN39, classic problem.... (I should know... been there, done that. Plus a professional technical translator as a wife.)
I agree the 1st June 2009 report translation could have been better.
Not sure whether the BEA thought it useful to have somebody 'nitpick' the text. It was aimed at professionals, not curious amateurs.
It's why I always try to read the originals, rather than the translations.....
Just having been reading through most of the English version of the 1st June 2009 report.
Some of it is clearly translated French, but none looks bad enough to lead to obvious misinterpretation by people familiar with the subject.
mm43 (re your earlier post), I would call it acceptable, but since I know French I can see where some of the terminology would have come from, even without having the French original next to the translation....
Whether it's as understandable to somebody without any French... I can't really judge.
HN39, classic problem.... (I should know... been there, done that. Plus a professional technical translator as a wife.)
I agree the 1st June 2009 report translation could have been better.
Not sure whether the BEA thought it useful to have somebody 'nitpick' the text. It was aimed at professionals, not curious amateurs.
It's why I always try to read the originals, rather than the translations.....
BEA Study of Indicated Speed Anomalies on A330/A340
HN39 and others,
I made the mistake, on April22 (Good Friday), of suggestinghere that the stall warning provided in Alternate Law is IAS-based. It is, as HN39 quickly pointed out, AoA based. (While I was out of circulation, that Part-1 thread was closed and locked, so am unable to edit the error.)
The fact that the stall warning is indeed AoA-based, as one would like, makes it all the more puzzling that as many of 9 of the 13 cases of unreliable airspeed indications leading to Alternate Law − analysed by the BEA in their Interim 2, 1.16.3 (the 13 best-recorded of the 36 incidents summarised in Appendix 7 of the same report) − involved stall warnings. Why puzzling? Because the same analysis seems to support the theory that these pitot problems lead to under-reading, not over-reading, airspeed indications. If the A/THR (not to mention the crew) is being led to believe that the IAS is falling when it isnt, why in level flight would these 9 cases involve the aeroplane finding itself at a high-enough AoA to trigger a stall warning?
Extracts from the BEA report, referred to by HN39:
The speed anomalies can be characterised by two distinct signatures:
- intermittent falls (peaks);
- fall followed by levelling off (continuous period).
[...] the lower speed limits recorded were below 100 knots.
[...]
In the cases studied, it is notable that:
- the airplane remained within its flight envelope...
Although the ambiguous expression peaks (has something been lost in the translation?) might be interpreted as indicating an intermittent rise in indicated speed in some cases, I dont think that is intended. The burden of the analysis seems to be of consistent under-reading of IAS. On the face of it, this and a high AoA in level flight seem to be mutually exclusive, but the BEA does not address that apparent inconsistency.
However, turbulence with a gross increase in normal acceleration (load factor) is one possible explanation.
Quote:
Turbulence was always recorded and reported. The levels felt by the crew varied from slight to strong. The recorded amplitude in recorded normal [acceleration] went from 0.75g/1.2g to 0.2g/1.9g.
Perhaps HN39 might be able to provide a graph of level-flight AoA versus load-factor, please?
Other inconsistencies of analysis results are apparent in the BEA report − always presented without comment. In this case of unreliable airspeed indications, the most striking vulnerability in the stall protection logic − if the BEA is telling us the whole story − is, as quoted by HN39:
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean configuration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
Given that an IAS of 100 kts − one of the figures recorded in the analysis − at FL350 is in the region of Mach 0.3, the inference is that a stall warning would not happen until an AoA of 10 degrees. On the other hand, in all cases studied, the airplane remained within its flight envelope...
As HN39 puts it:
the occurence of stall warning in the cases studied by BEA does not seem to match the higher threshold after loss of IAS.
I made the mistake, on April22 (Good Friday), of suggestinghere that the stall warning provided in Alternate Law is IAS-based. It is, as HN39 quickly pointed out, AoA based. (While I was out of circulation, that Part-1 thread was closed and locked, so am unable to edit the error.)
The fact that the stall warning is indeed AoA-based, as one would like, makes it all the more puzzling that as many of 9 of the 13 cases of unreliable airspeed indications leading to Alternate Law − analysed by the BEA in their Interim 2, 1.16.3 (the 13 best-recorded of the 36 incidents summarised in Appendix 7 of the same report) − involved stall warnings. Why puzzling? Because the same analysis seems to support the theory that these pitot problems lead to under-reading, not over-reading, airspeed indications. If the A/THR (not to mention the crew) is being led to believe that the IAS is falling when it isnt, why in level flight would these 9 cases involve the aeroplane finding itself at a high-enough AoA to trigger a stall warning?
Extracts from the BEA report, referred to by HN39:
The speed anomalies can be characterised by two distinct signatures:
- intermittent falls (peaks);
- fall followed by levelling off (continuous period).
[...] the lower speed limits recorded were below 100 knots.
[...]
In the cases studied, it is notable that:
- the airplane remained within its flight envelope...
Although the ambiguous expression peaks (has something been lost in the translation?) might be interpreted as indicating an intermittent rise in indicated speed in some cases, I dont think that is intended. The burden of the analysis seems to be of consistent under-reading of IAS. On the face of it, this and a high AoA in level flight seem to be mutually exclusive, but the BEA does not address that apparent inconsistency.
However, turbulence with a gross increase in normal acceleration (load factor) is one possible explanation.
Quote:
Turbulence was always recorded and reported. The levels felt by the crew varied from slight to strong. The recorded amplitude in recorded normal [acceleration] went from 0.75g/1.2g to 0.2g/1.9g.
Perhaps HN39 might be able to provide a graph of level-flight AoA versus load-factor, please?
Other inconsistencies of analysis results are apparent in the BEA report − always presented without comment. In this case of unreliable airspeed indications, the most striking vulnerability in the stall protection logic − if the BEA is telling us the whole story − is, as quoted by HN39:
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean configuration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
Given that an IAS of 100 kts − one of the figures recorded in the analysis − at FL350 is in the region of Mach 0.3, the inference is that a stall warning would not happen until an AoA of 10 degrees. On the other hand, in all cases studied, the airplane remained within its flight envelope...
As HN39 puts it:
the occurence of stall warning in the cases studied by BEA does not seem to match the higher threshold after loss of IAS.
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Hi Aileron Drag,
Thanks for the reply.
Could it not be designed to deploy when all 3 pitot tubes disagree simultaneously, or on the command of a pilot who has an intuition that something isn't right? Is the pitot tube system prohibitively expensive?
Again, thank you for indulging my query.
Thanks for the reply.
The RAT can be deployed automatically if hydraulic pressure fails. That is achieved by the total 'system' sensing a total loss of hydraulic pressure or a loss of all engine power.
In the case of iced-up pitots, the system would not 'know' that ice was causing erroneous readings. That is to say, the aircraft systems would need to be 'told' that the sensed 'q' was not to be trusted, because of ice.
But how could you tell if ice was causing the data to be unreliable? The systems would simply take the 'q' reading as correct, even if it was not.
It's difficult to imagine, therefore, how an automatically deployed standby pitot system would operate.
In the case of iced-up pitots, the system would not 'know' that ice was causing erroneous readings. That is to say, the aircraft systems would need to be 'told' that the sensed 'q' was not to be trusted, because of ice.
But how could you tell if ice was causing the data to be unreliable? The systems would simply take the 'q' reading as correct, even if it was not.
It's difficult to imagine, therefore, how an automatically deployed standby pitot system would operate.
Again, thank you for indulging my query.
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ChristiaanJ;
Thanks for your thoughts on the BEA's French to English translations. I have both forms of the reports on my computer, but being naturally lazy, I tend to accept the English version, and only if there is some doubt do I refer to the French original. As you pointed out the Google translation has improved immensely over the last year or so, and I can also use my rusty French to sort mixed tenses.
Thanks for your thoughts on the BEA's French to English translations. I have both forms of the reports on my computer, but being naturally lazy, I tend to accept the English version, and only if there is some doubt do I refer to the French original. As you pointed out the Google translation has improved immensely over the last year or so, and I can also use my rusty French to sort mixed tenses.
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Originally Posted by bearfoil
So if from ACARS the a/p dropped by command from (Computer) ADIRU disagree, then I can't imagine why auto would be allowed to re enter the room even if invited.
Originally Posted by HN39
Wouldn't the AP adjust its command before it results in an uncontrollable attitude?
I also think that if flight controls move with the amplitude adapted to low speed but the real airspeed is nowhere close to that, it can be very messy as henra put it.
Originally Posted by GerardC
You are right, we ALL take whatever weather deviation is needed, AND (preferably before deviating...) we send a message (HF or CPDLC) : "request deviation up to XX Nm due to weather".
Do not forget these routes are not as crowded as the NAT OTS area : after dozens of SA crossings (before and after AF 447 crash), I can't remember of much (if any) "deviation" chat on 123,45 (or 121,5).
Do not forget these routes are not as crowded as the NAT OTS area : after dozens of SA crossings (before and after AF 447 crash), I can't remember of much (if any) "deviation" chat on 123,45 (or 121,5).
Originally Posted by Shadoko
Is the AP going back automaticly if the a/c "think" the speed indications are right back?
Fron
Expense is not a decision gate here.
Measured reliability is the issue. The more systems you add the less reliable you are (less safe).
The typical design decision here is to have a backup that gives up on trusting screwy readings and still flies the aircraft. Else one would have to roll some dice to pick which reading is correct. I don't for an instant trust yet another probe with way less than 100% relibility (it only gets used once in a lifetime) that is expected to solve a unique problem.
Could it not be designed to deploy when all 3 pitot tubes disagree simultaneously, or on the command of a pilot who has an intuition that something isn't right? Is the pitot tube system prohibitively expensive?
Measured reliability is the issue. The more systems you add the less reliable you are (less safe).
The typical design decision here is to have a backup that gives up on trusting screwy readings and still flies the aircraft. Else one would have to roll some dice to pick which reading is correct. I don't for an instant trust yet another probe with way less than 100% relibility (it only gets used once in a lifetime) that is expected to solve a unique problem.
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French text
henra
"The French text does not point to a re-activation of the AP. It is merely stating that if the AP was connected before, it would have been disconnected as a consequence of this message."
With all due respect, not quite correct. The French text is simply stating that this message cannot be a consequence of a reset. Nothing more.
I'm French, so hopefully can still correctly interpret such sentences... And yes, French can be convoluted and confusing, sometimes on purpose!
"The French text does not point to a re-activation of the AP. It is merely stating that if the AP was connected before, it would have been disconnected as a consequence of this message."
With all due respect, not quite correct. The French text is simply stating that this message cannot be a consequence of a reset. Nothing more.
I'm French, so hopefully can still correctly interpret such sentences... And yes, French can be convoluted and confusing, sometimes on purpose!
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the occurence of stall warning in the cases studied by BEA does not seem to match the higher threshold after loss of IAS.
I think I understand how you see this, and I would like to risk an explanation.
The following quote from the BEA :
If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8
If none of the three Mach values is valid, a Mach value close to zero is used. For example, it would be of the order of 10° if the Mach was really at 0.3 and of 4° if the Mach was really at 0.8
A Mach 0.3 value can not be taken into account since none of the 3 Mach values is considered valid.
Do I make sense here ... ?
Originally Posted by DozyWannabe
... so disconnecting A/THR by moving the levers would be counter-intuitive. A/THR disconnect is achieved by pressing the red buttons on the side of the thrust lever handle.
The other standard A/THR disconnection procedure is to simply set all thrust levers to IDLE detent.
But to answer Shadoko question, moving the thrust levers cannot disconnect the AP.
Originally Posted by DozyWannabe
The computers will do exactly as the pilot asks of them and will not intervene unless the pilot is trying to do something dangerous
It won't make you an Airbus hater to admit it once for all.
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Current HF antennas
I mentioned the current VERY POOR ANTENNAS because they are also parallel to the vertical fin what further degrades the performance.
The shunting feed of VS is a good solution.
Itīs not "an antenna parallel to VS" as i thought.
Itīs a way to feed RF to the VS metal structure.
It has some disadvantages but works.
The efficiency could be good with a low SWR and a good ATU.
About effectiveness i reiterate the wire antennas used in old birds presented much higher ERP.
But the jet age (a/c speed) and space age (SAT) reduced the relative importance of HF to a/c comm.
____________________________________________________________ ___
Initial post
With the advances in other options HF had a decrease in it`s use.
The old wire antennas were much more effective. But with the jet age they were no longer used.
The "integrated" antennas are low efficiency radiators compared to the ones used in L188, C130`s and the older planes.
Electrically speaking the basis is the low radiation resistance and it`s lenght and position in the a/c.
If the commercial planes were equipped with good HF antennas the comm would be completely different.
After a trip with DEP 2 hours from now i will post a better reply with the laptop.
Well, letīs continue:
A short antenna has several disadvantages compared to one that is in the order of the magnitude of the wave lenght you are using.
At night in June in the middle of the night you may be required to use a low HF frequency.
When i worked doing HF equipt. maintenance in the rigs from PAA (12ACX, 4WTFA, etc) i remember the QRG 2910 KHz. An adequate lenght would be in the range of 60 ft. The Connies, etc used good antennas (long). With an old Collins ART-13 with derated tube to 100 W you had a very strong signal in AM.
Later with the introduction of SSB (USB, for aviation) it was possible to reduce the size paying some penalties in performance. Anyway the 707 fin and the LH wing antennas used by Lufthansa delivered good signals.
More and more the SIMPLE SYSTEM (using just "free" ionosphere layers) was replaced by repeaters, etc.
For example, this repeaters PLAYED A ROLE in MAC between N600XL and PR-GTD in Brazil.
I mentioned the current VERY POOR ANTENNAS because they are also parallel to the vertical fin what further degrades the performance.
It works but when you have 40+ years of HF experience in thousands of mobile operation (Land mobile, maritime mobile and air mobile) you has the obligation to say the HF antennas equipping the new planes (letīs say, after 707) ARE POOR.
Finally we must mention some EMI/EMC issues, a challenge for new composite "intensive" a/c.
I could assure that with 200W rms out, SSB and a "normal size antenna" your performance (comm and data) would amaze the younger crew.
Other considerations like S/N (signal to noise ratio), ionospheric propagation, etc i could comment further but this is not the issue here in the AF447 case.
Last edited by RR_NDB; 30th Apr 2011 at 22:54. Reason: Detected an error, a concept one and improved post
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JD-EE;
Just as a refresher, I've thrown some facts together based on what was actually happening at the time - NOTE : June 2009!
01:31:44 - RECIFE (126.5MHz) gave AF447 the ATLANTICO HF frequencies: 6649 or 5565kHz, then 6535kHz after the TASIL.
01:33:25 - AF447 contacted ATLANTICO on 6649kHz.
01:35:15 - AF447 reported to ATLANTICO that they passed INTOL 0133, at FL350, estimating SALPU 0148 and ORARO 0200. SELCAL - CPHQ.
HF Propogation
At 0200 UTC 2009-06-01 LKP to NATAL (Atlantico/Recife HF)
MUF = 7.0MHz, LUF = 2.0MHz, Solar Flux = 71, Hops = 1, Dist = 592NM.
Primary 6649kHz, Secondary 5535kHz
At 0200 UTC 2009-06-01 LKP to DAKAR (Dakar Oceanic)
MUF = 12.5MHz, LUF = 2.0MHz, Solar Flux = 71, Hops = 1, Dist = 1053NM
Primary 6535kHz, Secondary 5535kHz
Air to Air
The EUR - SAM Pilot to Pilot frequency is 123.45MHz.
The graphic shows the position of all four Air France flights at around 0210, and confirms that AF447 was well within VHF range of the other 3 aircraft.
Summary
Solar conditions were quiet ~ low activity. No lightning was observed by satellite detection, or reported by other flights that night in the Atlantic ITCZ. St. Elmo's fire was reported by a flight, and AF459 did report difficulties on HF, but whether that was due to static or just no reply from the station they called, I do not know.
The aircraft would be operating well below 10MHz and probably around 5MHz or even 3MHz at that hour of the night in June of 2008.
01:31:44 - RECIFE (126.5MHz) gave AF447 the ATLANTICO HF frequencies: 6649 or 5565kHz, then 6535kHz after the TASIL.
01:33:25 - AF447 contacted ATLANTICO on 6649kHz.
01:35:15 - AF447 reported to ATLANTICO that they passed INTOL 0133, at FL350, estimating SALPU 0148 and ORARO 0200. SELCAL - CPHQ.
HF Propogation
At 0200 UTC 2009-06-01 LKP to NATAL (Atlantico/Recife HF)
MUF = 7.0MHz, LUF = 2.0MHz, Solar Flux = 71, Hops = 1, Dist = 592NM.
Primary 6649kHz, Secondary 5535kHz
At 0200 UTC 2009-06-01 LKP to DAKAR (Dakar Oceanic)
MUF = 12.5MHz, LUF = 2.0MHz, Solar Flux = 71, Hops = 1, Dist = 1053NM
Primary 6535kHz, Secondary 5535kHz
Air to Air
The EUR - SAM Pilot to Pilot frequency is 123.45MHz.
The graphic shows the position of all four Air France flights at around 0210, and confirms that AF447 was well within VHF range of the other 3 aircraft.
Summary
Solar conditions were quiet ~ low activity. No lightning was observed by satellite detection, or reported by other flights that night in the Atlantic ITCZ. St. Elmo's fire was reported by a flight, and AF459 did report difficulties on HF, but whether that was due to static or just no reply from the station they called, I do not know.
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OK, how about "The computers, when behaving as designed - which is around 99.9(rec)% of the time - will do exactly as the pilot asks of them and will not intervene unless the pilot is trying to do something dangerous".
It wouldn't do you any harm to admit that Asseline could have screwed up, Jacquet always was on a hiding to nothing and you'll never get your yoke back, either - but it isn't going to happen.
Keep playing the subject though.
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RR_NDB
Inefficient and ineffective are two different ballgames.
As I pointed out inefficient does not matter at HF frequencies most used for aircraft unless the antenna is extraordinarily inefficient. The antenna matching elements are not that bad. (If they were they'd break down from the heat they generated.)
On transmit it matters a little. But, if communications works over the required ranges with required reliability, what matters the transmit inefficiency?
Now let's look at how inefficient the antenna may be. I believe the HF antenna is in the leading edge of the "plastic" vertical stabilizer. That makes it about 35' long or a quarter wave at about 7 MHz. So it will be quite efficient at 7 MHz or a frequency near that. (It may not run the full length of the VS leading edge.) The matching circuit would have less than 3 dB of loss even at 3 MHz. Matching might get "dicey" at frequencies near 14 MHz. But even that can be dealt with. End fed half wave antennas do remarkably well with a suitable matching circuit. Zeppelins used this form of antenna, hence that antenna's name, Zepp.
3dB loss is no big deal when power levels less than a watt are known to work over distances of 1000 miles at night on 75 meters unless the propagation goes REALLY sour, which it does during periods of solar minimum. That's why God made 160 meters, one might say. If dummy load leakage can make it from Idaho to Los Angeles quite legibly under quiet conditions that gives you an easy 1000 times more power for punching through the crud Ma Nature throws at you. Planes do not use 5 watt handi-talkie power levels. They run 100 W to very conservative 1 kW depending on the radio selected.
Now let's head bang on the antenna configuration. The trailing wire can be a really nice antenna with wonderful range, if you want to talk to somebody off to one side or the other of your plane. It's not "ideal" for talking directly ahead or to the rear. Drag is horrible, especially at Mach 0.8 type speeds.
The slightly newer style of a wire running from behind the cockpit to an attachment on the tail is required for tin cans. Since the active element is very close to the counterpoise the efficiency can suffer dramatically. And it has high drag. It also suffers from some of the same directivity problems. They'd not work all that well to the direct front or rear. Finally the close proximity to the ground plane makes the antenna pretty much of an NVIS device, Near Vertical Incidence Signaling - short range regional. It hurts the long distance capabilities.
Embedding it in the tail is rather nice as a configuration. It is near vertical with, as a result, a nearly circular low elevation angle radiation pattern. In other words it is a really good long distance antenna. It works well in all directions including the critical direct forward and rear directions. It suffers a little for lack of a nice ground plane unless some screening is included around the base of the antenna. I'd imagine they've worked out something. On the whole 50% efficient is probably good enough. (Demonstrably it is good enough for transmit. And for receive it's efficient enough the inefficiency could even be increased to minimize sensitivity issues in a very high noise level environment.)
So, the antenna is effective. And it costs less than the longer antennas. So arguably it is more cost effective than the alternatives and not nearly as bad as a 9' whip on the back of an automobile. Now THAT is what I call inefficient.
Now that I've slammed this one into the ground (Earth for you British if you want the intended pun) can we quit carping about the antennas? I don't want to threaten dragging out various handbooks and playing pedagogue WAY off topic to prove my points. (The RF I am sure about. The precise VS dimensions and antenna dimensions within the VS structure are rough based on what information I could find in a 3 minute search using Google. Tail height is 9.3 meters. It's swept back some so I gave it a few more feet for the length of the leading edge.)
Inefficient and ineffective are two different ballgames.
As I pointed out inefficient does not matter at HF frequencies most used for aircraft unless the antenna is extraordinarily inefficient. The antenna matching elements are not that bad. (If they were they'd break down from the heat they generated.)
On transmit it matters a little. But, if communications works over the required ranges with required reliability, what matters the transmit inefficiency?
Now let's look at how inefficient the antenna may be. I believe the HF antenna is in the leading edge of the "plastic" vertical stabilizer. That makes it about 35' long or a quarter wave at about 7 MHz. So it will be quite efficient at 7 MHz or a frequency near that. (It may not run the full length of the VS leading edge.) The matching circuit would have less than 3 dB of loss even at 3 MHz. Matching might get "dicey" at frequencies near 14 MHz. But even that can be dealt with. End fed half wave antennas do remarkably well with a suitable matching circuit. Zeppelins used this form of antenna, hence that antenna's name, Zepp.
3dB loss is no big deal when power levels less than a watt are known to work over distances of 1000 miles at night on 75 meters unless the propagation goes REALLY sour, which it does during periods of solar minimum. That's why God made 160 meters, one might say. If dummy load leakage can make it from Idaho to Los Angeles quite legibly under quiet conditions that gives you an easy 1000 times more power for punching through the crud Ma Nature throws at you. Planes do not use 5 watt handi-talkie power levels. They run 100 W to very conservative 1 kW depending on the radio selected.
Now let's head bang on the antenna configuration. The trailing wire can be a really nice antenna with wonderful range, if you want to talk to somebody off to one side or the other of your plane. It's not "ideal" for talking directly ahead or to the rear. Drag is horrible, especially at Mach 0.8 type speeds.
The slightly newer style of a wire running from behind the cockpit to an attachment on the tail is required for tin cans. Since the active element is very close to the counterpoise the efficiency can suffer dramatically. And it has high drag. It also suffers from some of the same directivity problems. They'd not work all that well to the direct front or rear. Finally the close proximity to the ground plane makes the antenna pretty much of an NVIS device, Near Vertical Incidence Signaling - short range regional. It hurts the long distance capabilities.
Embedding it in the tail is rather nice as a configuration. It is near vertical with, as a result, a nearly circular low elevation angle radiation pattern. In other words it is a really good long distance antenna. It works well in all directions including the critical direct forward and rear directions. It suffers a little for lack of a nice ground plane unless some screening is included around the base of the antenna. I'd imagine they've worked out something. On the whole 50% efficient is probably good enough. (Demonstrably it is good enough for transmit. And for receive it's efficient enough the inefficiency could even be increased to minimize sensitivity issues in a very high noise level environment.)
So, the antenna is effective. And it costs less than the longer antennas. So arguably it is more cost effective than the alternatives and not nearly as bad as a 9' whip on the back of an automobile. Now THAT is what I call inefficient.
Now that I've slammed this one into the ground (Earth for you British if you want the intended pun) can we quit carping about the antennas? I don't want to threaten dragging out various handbooks and playing pedagogue WAY off topic to prove my points. (The RF I am sure about. The precise VS dimensions and antenna dimensions within the VS structure are rough based on what information I could find in a 3 minute search using Google. Tail height is 9.3 meters. It's swept back some so I gave it a few more feet for the length of the leading edge.)
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mm43, thanks for the actual numbers. The MUF towards DAKAR surprises me for how high it is. I guess I'm too used to conditions around LAXish areas. I'd expect DAKAR to communicate with the plane nearer to 11MHz given the MUF cited. But 6-7 MHz would work OK. (And the antenna would be a little happier.)
I know this has been hinted at previously, but this discussion does make me think about the qualities required for pilots flying these aircraft While the pilots are trained in these computerized flight control systems and have a degree of familiarization with them, do they actually have a deep understanding of them? If not, then is desirable to be operating aircraft without a comprehensive understanding of these systems?
Previously we talked about "airmanship". And that quality is clearly vitally important (witness Sully's landing in the Hudson and even the last seconds of BA038). But do pilots today need more than this? Do they also need to be masters of current technology FBW and automated systems? And if so, is it possible to be that, and to be a good pilot also? Or is that skill set asking too much of one profession?
It appears inevitable that automation will increase. And if commercial air travel is safer than ev before, then perhaps we are on the right track overall (despite the occasional disaster such as this). Perhaps we have to be pragmatic and look at the big picture.
Careers evolve over time in the face of new technology, and I guess that is progress. But I wonder if flying a jet today is still the same job as before, or whether it is something new and fundamentally different. And whether pilot training today gives people the skills they will need.
Or do we need to go back to the days of a flight engineer? But this time, someone trained in computers and systems and instrumentation rather than engines.
I know you can argue that you can't over-react to a single incident. But QF072 could have ended very differently. And that Qantas 380 with that engine problem out of Singapore - true it started as a mechanical failure, but the crew was overwhelmed with the amount of information presented to them and you wonder how this would have turned out with a "normal" crew rather than a lot of experienced check pilots.
Previously we talked about "airmanship". And that quality is clearly vitally important (witness Sully's landing in the Hudson and even the last seconds of BA038). But do pilots today need more than this? Do they also need to be masters of current technology FBW and automated systems? And if so, is it possible to be that, and to be a good pilot also? Or is that skill set asking too much of one profession?
It appears inevitable that automation will increase. And if commercial air travel is safer than ev before, then perhaps we are on the right track overall (despite the occasional disaster such as this). Perhaps we have to be pragmatic and look at the big picture.
Careers evolve over time in the face of new technology, and I guess that is progress. But I wonder if flying a jet today is still the same job as before, or whether it is something new and fundamentally different. And whether pilot training today gives people the skills they will need.
Or do we need to go back to the days of a flight engineer? But this time, someone trained in computers and systems and instrumentation rather than engines.
I know you can argue that you can't over-react to a single incident. But QF072 could have ended very differently. And that Qantas 380 with that engine problem out of Singapore - true it started as a mechanical failure, but the crew was overwhelmed with the amount of information presented to them and you wonder how this would have turned out with a "normal" crew rather than a lot of experienced check pilots.
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Hi, JD-EE
I will start now and continue later editing my answer:
1) The HF antennas are not located in "no conductive VS".
2) They run near and in PARALLEL to metal. Much better would be at right angles (like the 707 Collins design)
2a) Correction: They RF excite the VS in a "shunt configuration". A driven thick wire is connected to VS at an height ranging from 3 to 9 ft or so. This is not the "main" antenna. But radiates (when transmitting) some of the energy because of high RF current circulation. Iy actually supply RF energy to VS. Can be considered part of the coupling circuitry (Z match).
3) The signal to the rear IS MUCH WEAKER simply because no "return=ground plane" behind VS.
4) Drag for a trailing short wire (letīs say 60 ft) i donīt think would be a problem. Ok, you have APU, etc. But iīm not advocating the use of the System Amelia Earhart had in the Electra or the reel wire in the C47, etc. What you say about trail a 30,000 ft like the 707/CFM56 of USN for subs? Ok, it oscillates and USN paid for a damper for it,
To be continued...
Rgds, appreciated in commenting
I will start now and continue later editing my answer:
1) The HF antennas are not located in "no conductive VS".
2) They run near and in PARALLEL to metal. Much better would be at right angles (like the 707 Collins design)
2a) Correction: They RF excite the VS in a "shunt configuration". A driven thick wire is connected to VS at an height ranging from 3 to 9 ft or so. This is not the "main" antenna. But radiates (when transmitting) some of the energy because of high RF current circulation. Iy actually supply RF energy to VS. Can be considered part of the coupling circuitry (Z match).
3) The signal to the rear IS MUCH WEAKER simply because no "return=ground plane" behind VS.
4) Drag for a trailing short wire (letīs say 60 ft) i donīt think would be a problem. Ok, you have APU, etc. But iīm not advocating the use of the System Amelia Earhart had in the Electra or the reel wire in the C47, etc. What you say about trail a 30,000 ft like the 707/CFM56 of USN for subs? Ok, it oscillates and USN paid for a damper for it,
To be continued...
Rgds, appreciated in commenting
Last edited by RR_NDB; 1st May 2011 at 01:53. Reason: Conceptual error correction
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HF Antennas
Modern HF antennas and couplers in commercial aircraft are quite different from those described above. By modern, I mean 1968 and newer.
All the modern airliners use current-fed notch/slot antennas. They amount to a battery cable attached to the dorsal or tail fin; very low voltage, very high current. The whole plane becomes the antenna. Yes, if designed and maintained correctly, it is highly efficient.
The best commercial antenna flying still today was on the first DC-10, and continued on the MD-11. It was designed by Ben Hornby, an ex-pat Brit whose learning was on the VC-10 or something. The DAC tri-jet has the ideal location for the shunt fed antenna, just below the #2 engine, for really good coupling. The L-1011 was not so good, and Hornby had to help them make it work.
Boeing did not switch to the leading edge shunt antenna until pressured by airlines some years later. Their first was in the 727, which design they bought from Eastern Airlines. Boeing still did not understand shunt antennas a dozen years later, so the 767 had terrible HF for years, maybe still.
Airbus adopted shunt antennas in the fin from the beginning. I don't know how well they understand the dynamics, but feeding hundreds of amps into a carbon fiber fin makes me wary. That said, I have no direct experience with A/B HF antennae. AA587 and prior events it highlighted really had me wondering.
Airline HF is 400W P-P, BTW, and always USB, upper sideband. The antenna slot in most airliners is no more than a meter long.
Is this pertinent to AF447? Only regarding SelCal, I guess.
GB
All the modern airliners use current-fed notch/slot antennas. They amount to a battery cable attached to the dorsal or tail fin; very low voltage, very high current. The whole plane becomes the antenna. Yes, if designed and maintained correctly, it is highly efficient.
The best commercial antenna flying still today was on the first DC-10, and continued on the MD-11. It was designed by Ben Hornby, an ex-pat Brit whose learning was on the VC-10 or something. The DAC tri-jet has the ideal location for the shunt fed antenna, just below the #2 engine, for really good coupling. The L-1011 was not so good, and Hornby had to help them make it work.
Boeing did not switch to the leading edge shunt antenna until pressured by airlines some years later. Their first was in the 727, which design they bought from Eastern Airlines. Boeing still did not understand shunt antennas a dozen years later, so the 767 had terrible HF for years, maybe still.
Airbus adopted shunt antennas in the fin from the beginning. I don't know how well they understand the dynamics, but feeding hundreds of amps into a carbon fiber fin makes me wary. That said, I have no direct experience with A/B HF antennae. AA587 and prior events it highlighted really had me wondering.
Airline HF is 400W P-P, BTW, and always USB, upper sideband. The antenna slot in most airliners is no more than a meter long.
Is this pertinent to AF447? Only regarding SelCal, I guess.
GB