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Guest Martin

Unreliable instruments

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Guest Martin

I just wrote a long post, and naturally it disappeared, and in the process the clock managed to turn about 30 minutes forward, and it's late. But I'll write this down anyway. Bear with me.Among threads about dirty Air France planes and new meals at Air China I stumbled upon a link to the transcript of Aeroperu flight 603], the 757 that crashed into the ocean due to taped over static ports after taking off from Lima, Peru. It was one of the scariest accident transcripts I've read, mostly because of the unbelievably bad CRM of the crew, but also because I began wondering if you would really be in that much trouble with no working altimeter, VSI or ASI.I decided to try it out in PIC - to fly without working ASI, altimeter and VSI at night with no visual reference.I failed the pitot static system and the altimeter using the FS System Failures menu (since there are no separate failures for this in PIC). This is a little unrealistic since when the instruments are failed it's quite obvious; the ASI is stuck at 30 even though you're roaring down the runway. They aren't really unreliable.A/P, A/T and F/D all became useless at once, naturally.I still had the GS readout on the EADI and even a TAS readout on the FMC PROG page 2. The radio altimeter, map display on the EHSI, and ILS were also working (naturally), so there really wasn't much of a problem, and since wind was calm using ground speed was easy. I flew it down the ILS, using the RA and glide slope for altitude reference and made a landing that was totally OK, but perhaps a little fast.Next, I disabled the Air Data Computer in the PIC menu. Now the FMC was useless and no flight plan data was displayed on the EHSI, but GS was still available on the EADI. It was no big difference to before, so now I turned off the three IRUs. EADI and EHSI went black of course. I restored the IRS in ATT mode so I now had attitude data on the EHSI, but no EHSI and no ground speed. (I didn't remember exactly how to input the heading into the IRS to make it show on the EHSI in ATT mode, so I didn't bother.) Now it was a little trickier, but still far from impossible. Using the good (?) old whiskey compass and RDMI to get an idea of where I was. (In the real world, ATC would of course have been there to help out.) The LOC deviation needles on the EADI and ADI only don't give a very good idea of your relation to the localizer beam though, so I overshot the localizer several times to the left and right, but managed to follow the glideslope without too much trouble. I broke out at around 300 ft and managed to yank the 767 onto the runway, a little late and a little fast but still a landing. Stopping with reverse and manual braking was no problem (autobrakes were inoperative since the IRS was turned into ATT).Now to some questions:How well does PIC simulate situations like these? Unreliable airspeed and altimeters aren't implemented (perhaps an FS problem?), so I imagine it would be a lot more challenging if you had an ASI showing 350 knots rather than one showing 30. When it shows 30 and you're flying you know it's lying, and it's thus reliable ;-).I didn't get any overspeed or other warnings, and I guess that's because the ASI stayed at 30 knots throughout the flight - even when I went to over 400 TAS at about 10,000 ft to try it out.Is it realistic to have a TAS readout on the PROG page when the ASI is inoperative? How dose that calculation work?Is it correct that the IRS does not only calculate lateral position but also vertical (i.e. altitude :-))? If so, is this, or can this, be used if all other altitude sources fail?And how could the Aeroperu crew not recognise that the instruments were unreliable? (I suppose they did at some point along the line, but still seemed to rely on the instruments.) Why did they never check the radio altimeter? Why didn't they check the GS readout? They didn't have problems with the IRS or ADC, so that should have worked, as well as the FMC, so they could have entered the approach into Lima again (assuming they had it programmed) and then they wouldn't need to rely so heavily on vectors from ATC - at least they would have a very good idea of where they were in relation to the airport.Martin, needing sleep, once again discovering that any resemblance of a sleepy smiley has been removed, so I'll have to substitute this one: :-dohIt's a lot like life and that's what's appealing[/i

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Guest Martin

Interesting topic, Martin! Deserves a bump.It's a lot like life and that's what's appealing

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Guest Ian_Riddell

So many questions, I don't really know where to start, Martin 8-) I also read the transcript and I couldn't think of any solutions to the problems they had.Here's just a few pointers to give you some idea of what the pilots were up against....The ADC's get their information from the pitot/static system, so the ADC's would have been presenting false information to the crew.ATC was giving them their altitude but, more than likely, ATC were simply looking at the (false) altitude data the aircraft was sending them. The ATC systems on the aircraft simply transmit the airplane's ADC altitude data.TAS is generated by the ADC's.EHSI wind data is partly generated by the ADC's.IRS groundspeed is not much use to you if you don't know what altitude you're at or what the winds are.V/S data would be unreliable as this is a combination of IRS and ADC data (The IRS provides instantaneous V/S, but the ADC's provide long term correction).Radio Altimeters only have a range of 2500', so if the airplane stalled or was going at high speed (downwards) at an altitude slightly above this, the crew would have had little time to react to any GPWS callouts. Don't forget also, for some warnings, the GPWS system gets its data from the ADC's.The false information on their displays would have been hard to ignore. I'm sure they would have wanted to believe some of it. They knew that at least some of their air data was false, but how would they have known which parts were correct? There would have been no failure flags to tell them.I'm very surprised that the covers were not discovered on the pilots' walkaround, but, of course, there is no excusing the actions of the engineers. The blame falls fairly and squarely on their shoulders. Long, highly visible streamers should have been attached to any covers/patches fitted to the pitot/static system during maintenance.I doubt anyone would have been able to recover from this scenario (especially with no external visual cues).Hope this helps?Cheers.Ian.

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Guest Martin

Thanks Wade, very useful chart (although I hope I don't have to use it). Made me look in my manual also, and in deed it also had tables for unreliable airspeed, but in kilograms, so now I have both.MartinIt's a lot like life and that's what's appealing

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Guest Martin

Thanks for your input Ian.I am thinking that with tables for unreliable airspeed/vertical speed, which I assume the pilots must have had, and with a working map view, radio altimeter, and ILS receivers they should have had a chance.West of Lima airport is only ocean, so with the map view I guess they should have had a good idea of their position. Using the tables they could have tried to descend low enough to receive the RA readout, and then turned onto the ILS, and make a landing.Perhaps I'm assuming too much, without knowing all the circumstances. (I don't know if the weather was down to minimums, or if any equipment was inop on the aircraft from the beginning.)But it still seems to me that it shouldn't be a totally hopeless scenario, if they pilots can just be trained to ignore the unreliable instruments, including turning off F/D and A/T. It seems that these pilots were not trained for a situation like this.MartinIt's a lot like life and that's what's appealing

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It's always easy to second guess after the fact.However ;), according to the CVR, it seemed like they couldn't make up their mind whether to use the autopilot / auto-throttle, at the same time, they kept saying that it wasn't working.30 minutes into the tape they are still saying that they didn't have accurate airpseed indications. You'd think ??? that after 5 mins or less they would have realized they had no airspeed ind, and would have just just flown power+attitude ??? I.E. 29mins the pilot is still trying to use "flight level change".>IRS groundspeed is not much use to you if you don't know what altitude >you're at or what the winds are.Well, you might think it would be accurate enough to prevent an overspeed or stall ?? Again, you'd think they would have a good ideaof the power settings that would produce an appproximate (+- 50kts ???)Since their navigation instruments apparently were still working, they were talking to ATC, they were only 40 miles from the airport that had an ILS..... had a lot things going for them.Pitch black at night in a 757, with pitot failure, admittedly terrifying , but that is what training and 5-10,000 hrs of experience is supposed to be for ... isn't it ? And why there are two crew-members.I think they just panicked and forgot all training under the heat of the moment. ...I might have done the same thing.

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Guest Ian_Riddell

"Again, you'd think they would have a good idea of the power settings that would produce an appproximate (+- 50kts ???)"I really don't see how the charts would help. There are simply too many unknowns. For example, if your airplane was 3 degrees nose up and weighed 240,000lbs and your engines were at 94%N1, are you sinking or climbing? At 30,000', you may be flying level, but at 5,000', you could be doing something completely different (and your speed could be slowly or even rapidly changing). You simply don't know (until you feel you are stalling or parts of your aircraft start breaking off because of overspeed). :-)Add to this confusion, the false(?) aural warnings screaming (they are loud) in your ears. There are many aural warnings (including overspeed) which cannot be cancelled (other than by pulling the CB's for the aural warning system). If you pull the CB's, you may not hear other warnings, such as Rad Alt altitude callouts. If the stickshaker is falsely activated, this can be very distracting. Add to this the possibility that the pressurisation system is malfunctioning and you have a situation which probably two pilots are simply not able to handle. I don't think we'll realize what they were faced with until it happens to us in real life (or the sim instructor feeds in false air data to both the Captain's and F/O's instruments _unannounced_ in training).Looking forward to comments from pilots who have trained for this scenario :-)Cheers.Ian.P.S. By the way, ADC data is also sent to the EEC's on some aircraft types. There may be engine fault messages also being generated as a result of faulty air data.P.P.S. Which instruments are pilots taught to believe if the Captain's and F/O's agree with each other and the Standby Instruments don't agree?

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Guest Martin

>I really don't see how the charts would help. There are >simply too many unknowns. For example, if your airplane was >3 degrees nose up and weighed 240,000lbs and your engines >were at 94%N1, are you sinking or climbing?This information tells you that you are most likely climbing. You should know what you want to do, i.e. if it's right after takeoff you want to climb. Look up the correct body attitude in the chart, and fly that attitude at the thrust setting from the chart.If you are close to the ground, the RA readout is of course available.I just had a look in the QRH and in deed it says you should do what I did. :-) That is use the tools you have: the charts for approximate attitudes and thrust settings, the RA when you're close to the ground, GS readout for approximate speeds, and preferably ILS when landing.Of course the danger of distraction from all the warnings remains, and the danger of trusting the instruments. But with proper traininig, I think pilots should be able to ignore the warnings enough to be able to fly the plane, and to be able to tell when instruments are going nuts. I'm not saying that it should be easy, but that we shouldn't just accept to be doomed if faced with this problem, and I don't think we would be. "Good" airlines train for this don't they? You should know for instance that a RUDDER RATIO message may be an indication of unreliable airspeed.>P.S. By the way, ADC data is also sent to the EEC's on some >aircraft types. There may be engine fault messages also >being generated as a result of faulty air data. And don't forget inaccurate EPR/N1 limits and reference bugs.>P.P.S. Which instruments are pilots taught to believe if the >Captain's and F/O's agree with each other and the Standby >Instruments don't agree? I would assume that they should trust the primary instruments if they agree and the standbys don't. That seems most logical to me.MartinIt's a lot like life and that's what's appealing

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Guest Ian_Riddell

">This information tells you that you are most likely >climbing."Is "most likely" good enough, Martin? I'm no aerodynmicist, but couldn't you be flying straight and level and accelerating instead of climbing (with this same combination of attitude and thrust). It won't take you long to get a real overspeed in this situation. >>P.P.S. Which instruments are pilots taught to believe if the >>Captain's and F/O's agree with each other and the Standby >>Instruments don't agree? >>I would assume that they should trust the primary >instruments if they agree and the standbys don't. That seems >most logical to me. I don't remember all the details of the incident, but if, say, the captain's and F/O's static ports had been left covered by maintenance... and the Standby Altimeter had not been covered, the Captain's and F/O's instruments would agree (but would be wrong) and the Standby Instruments wouldn't agree (but would be correct). You don't know what's really happening until you stick your head out of the window and have a look at the probes/ports (a little difficult at 250kts).I guess the only way to reproduce this scenario is to use bits of masking tape to cover your windows (for total lack of visibility) and air data displays on your computer monitor.... and fly for 15~20 minutes and guess at what altitude your aircraft will be and what speed you are flying). Of course, you will have to switch off the sound, so you don't get correct aural warnings. And then have someone give you a false height reading every now and then just to confuse you. I'd say even experienced pilots would be totally disoriented after this time.Any takers? :-)Cheers.Ian.

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Guest Martin

>">This information tells you that you are most likely >>climbing.">>>Is "most likely" good enough, Martin? I'm no aerodynmicist, >but couldn't you be flying straight and level and >accelerating instead of climbing (with this same >combination of attitude and thrust). It won't take you long >to get a real overspeed in this situation. If the instruments become unreliable, you would have a pretty good idea of what speed/altitude you were at from the start: If you have just taken off, you know you are not at 30,000 ft. And if you have been cruising at FL310, you know you are not below 5,000. Thus you will have a pretty good idea of what attitude and thrust setting you want to fly for the particular phase of flight.Perhaps you cannot be certain that you are climbing, but you can look at the chart to know what attitude you want in order to climb. It might not be perfect, but it's certainly better than assuming you're doomed from the start.>I don't remember all the details of the incident, but if, >say, the captain's and F/O's static ports had been left >covered by maintenance... and the Standby Altimeter had not >been covered, the Captain's and F/O's instruments would >agree (but would be wrong) and the Standby Instruments >wouldn't agree (but would be correct). You don't know what's >really happening until you stick your head out of the window >and have a look at the probes/ports (a little difficult at >250kts).Of course you are correct Ian. But remember, that if the captain's and F/O's instruments don't agree, the pilots are taught to trust the standby instruments. But how do they know that one of the pilots' instruments are not correct and the standbys are?>Any takers? :-) The "problem" with this little exercise is that it can't be realistically done in PIC. The instruments never become unreliable, they just stop at whatever reading they show when they fail, so you'll know right away that they are not working, and you won't be confused. There won't be any warnings - at least not stall or overspeed ones - unless perhaps if the instruments fail during a stall or overspeed. (I haven't checked that.)MartinIt's a lot like life and that's what's appealing

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Guest

Good discussion. There is one other factor in the real aircraft that cannot be simulated - fear. My guess is this probably the biggest barrier to the thorough analysis you fellas have gone through.Good stuff, though. Ideally, the flight crew should be able to work though all this, but a battle plan rarely survives contact with the enemy. :)

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Guest ca787546

Hi Martin, I'd like to begin by telling you that as bad as it seem, that crew didn't have terribly bad CRM, as you said. I am an A320 FO for a Peruvian Airline (it's not TACA, so that narrows it down if you do your research). I also happen to be a close friend of the Accident investigator who was in charge of the Aeroperu 757 crash. I am friends with many people that flew with both the captain and the fo of the Aeroperu flight in many occassions. And well, as if this is not enough, my mother was a FA for aeroperu for nearly 20 years before joining the AirCanada team in the 90s.To make a long story short... it is quite easy to find ways to get around the accident, analyze it in front of your computer, sipping a nice cup of tea/coffee, with all the manuals nicely laid in front of you, standing to pee every now and then, and then thinking about it some more. You also have to consider that when you go into the sim to try it, you go with the mindset of going into a pre-determined failure, which leaves you mentally and emotionally prepared for what is in store for you. You have to realize that back when the accident occurred, the boeing 767/757 unreliable speed indication in the QRH was pretty much non-existent as well... so the people at boeing, including engineers and test pilots with quite a bit of experience under their belts, hadn't put much thought into this emergency/abnormality either! So the crew was left with a terribly confusing situation at hand. You also have to consider that it was dark (cockpits at night are quite dark, specially if you're flying IFR (you don't want to have your dome light on unless you're in cruise), unfriendly and full of alarms, strange indications. This puts a fairly high level of stress on the crew... thus it wasn't CRM that I'd consider to be the problem, but more than that, it was simply an emergency that hadn't really been considered fully and deserved more studying... it is VERY similar to what happened with the swissair flight off the coast of NovaScotia... you wouldn't believe the changes that have occurred to our smoke/fumes checklist in the airbus thanks to that accident (and that wasn't even the same type of aircraft!). They still change our smokes/fumes checklist every now and then.. that's to show you that even years later, they're still working at it...hmmm, what else can I add... ok, I'll wrap it up re-inforcing the first point about being at home relaxing and flying FS being a bit different... I practice engine failures every 6 months, not 1,2 or three... but quite a few more just for recurrency in the full motion level-d flight sim, part of the airline requ's... nevertheless, the first time one of our engines went dead in the real thing... hmm... let's just say you level of stress and adrenaline is slightly higher (catch my drift?? :)... Anyway, try to figure out what you'd tell over 150 people when that happens... while trying to fly the aircraft and deal with the problem, and what to let the FAs know about the problem...Well, that is all I had to say, sorry I just rambled on, it probably seems chaotic and in a complete state of disorder, but I just didn't feel like proof-reading at all.Happy landings everyone,DN

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Guest ca787546

Hi, sorry, just had to add an answer to your question of why they didn't recognize the unreliable information.Here's the explanation:The static port detects the barometric pressure outside to give an altitude reading. Now, if I put a piece of tape on the static port, this is what will happen. As I climb, because the pressure outside is lower than the one inside the static port, the air will be able to escape, indicating a climb, like it is suppossed to. Right?? So far, so good, no evidence of trouble. The problem is once you level off, or the pressure gradient flattens out, or you descend... what happens then? the process is reversed, the pressure outside will be higher than the pressure inside... that creates a suction in the static port which sucks the piece of tape close and tight in the the port.. not letting any air in or out, thus maintaining a false altitude indication. That makes it rather difficult to notice. Boeing has the 80'knot call-out and the positive-rate call-out. This took place normally on the flight... that shows you that they didn't find any problems on take-off, and nothing showed up on the EICAS..Finally, to answer another question, is about the ATC. ATC in Lima is still done as procedural. This means, NO radar vector can be given legally since the Radar is not certified. Ussually the radar gets returns from the transponder-S or -C, which means the altitude the controller gets is the one the airplane sends (which is the same the crew gets in their instruments) which certainly adds to the confusion.... Now... if you're in an indicated level flight as told by your VSI, ALT, ATC, and your speed is increasing like crazy??? what is wrong? the alt? the IRS? the AH? the SI? you know?? it gets quite complex... I think you're very much right at saying that the aircraft could be landed safely. It could've! No doubt about it. But unfortunately for the people in that aircraft, and pretty much many other accidents (90% or more are due to human factors) it wasn't landed safely... I'll leave that there,Happy Landings,DN

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This was discussed somewhere before but I can't find the forum. Someone there had actually gone through it. Maybe someone could chime in.JimCYWG

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