axmiha
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Joined: Tue Jun 13, 2017 4:02 am

Aeroperu crash question for pilots

Sat Sep 09, 2017 1:35 am

Hello, I'm not a pilot, just an enthusiast (?) and I do sceneries for X-Plane - and fly some planes in X-Plane, so that's the extent of my knowledge (near zero). About the Aeroperu crash, that went down because the pitot static ports were covered and the crew didn't have correct speed and altitude indications, my question is; how did they take off if they didn't have speed indications? How did they call out the speeds - 80knots, V1 etc - if the pitots were covered? Is there another instrument that indicates the speed while they're on the ground, or something like that? If they called out the speeds, they did have speed indication during takeoff, but how is that possible if the pitots were covered? This has been a mystery to me and I can't find answers anywhere.

Thank you.
axmiha
 
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RetiredNWA
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:01 am

Aeroperu 603 is an interesting accident and an unfortunate one.

The static ports were covered with clear adhesive tape whilst the aircraft was undergoing a wash-up. When air is trapped in static ports, very basically, it will produce erroneous airspeed, altitude and vertical speed readouts.

Unfortunately, the crew of Aeroperu 603 was hit with a barrage of warnings on the EICAS, minor and major, and, they were unable to work through the issues and eventually lost the airplane.

If they had relied on their radar altimeter and flap position gauge they could have interpolated their airspeed based upon the flap blowback and their altitude would have been accurately displayed on the radar altimeter.

This post probably belongs in tech/ops - feel free to PM me for a more comprehensive explanation.
 
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VirginFlyer
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:14 am

I assume this is the accident you are referring to: https://aviation-safety.net/database/re ... 19961002-0

It was only the static ports which were covered, not the pitot tubes. To understand The significance of this, let me briefly describe the way the pitot-static system works:

A static port measures the static air pressure (that is the air pressure outside the aircraft), and are designed so that in normal flight the air flow is parallel to the port and not applying any dynamic pressure, while the pitot tube measures the total air pressure (that is the static pressure plus the dynamic pressure which is created as a function of air speed). Altimeters work by comparing the static pressure to a set pressure, so if the static port becomes blocked, the altimeter will read a constant altitude (whatever the altitude was when the port became blocked). Vertical speed indicators work similarly, but use a metered leak to allow it to indicate rate of change of static pressure, so if the static port becomes blocked, then vertical speed indicator will read zero. Air speed indicators work by comparing the total pressure from the pitot tube with the static pressure - the difference between them is the dynamic pressure; in other words the air speed. With a blocked static port, your air speed indicator will still show air speed, and so long as you maintain the altitude where the blockage occurred, this will be accurate. However if you climb, the blocked static port will mean the pressure in the static system is higher than reality, so the difference between pitot and static will be less than it should be, and the air speed will underread. Likewise if you descend, the airspeed will begin to overread.

Unfortunately all this means that if your static port becomes blocked on the ground, you won't get any indications of a blocked static system through the instruments until the aircraft becomes airborne (or if there has been a sufficient pressure change on the ground since the blockage, you may see the air speed indicator being non-zero at rest). This is what happened in the case of the AeroPeru accident - the issue became apparent once they were airborne.

V/F
"It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens." - Bahá'u'lláh
 
cedarjet
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:14 am

RetiredNWA doesn't really answer the question though. Yes there are plenty of ways of establishing your speed and altitude (flap blowback speeds not being the first that springs to mind — pitch and power being the obvious one) but the mystery remains, unreliable indications on the takeoff roll should have meant a low speed abort. I don't know why they took it into the air.

Same questions regarding an almost identical crash, Birgenair 301:
https://en.m.wikipedia.org/wiki/Birgenair_Flight_301
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
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RetiredNWA
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:26 am

cedarjet wrote:
RetiredNWA doesn't really answer the question though. Yes there are plenty of ways of establishing your speed and altitude (flap blowback speeds not being the first that springs to mind — pitch and power being the obvious one) but the mystery remains, unreliable indications on the takeoff roll should have meant a low speed abort. I don't know why they took it into the air.

Same questions regarding an almost identical crash, Birgenair 301:
https://en.m.wikipedia.org/wiki/Birgenair_Flight_301


I offered the original poster, and, anyone else with an interest or opinion the option to private message me for a more comprehensive answer.

You armchair aviators and CEO-wannabes criticize everyone attempting to answer a legitimate question posed by an interested party, or, those who have learned the art of brevity.
 
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VirginFlyer
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:33 am

cedarjet wrote:
Same questions regarding an almost identical crash, Birgenair 301:
https://en.m.wikipedia.org/wiki/Birgenair_Flight_301

The Birgenair accident with a blocked pitot is a different kettle of fish - the erroneous indications presented themselves during the take off run, and there was an opportunity to reject the take off. A pitot blockage is much more noticeable on a take off run than a static blockage (which in many cases will be completely unnoticeable until the aircraft becomes airborne)

V/F
"It is not for him to pride himself who loveth his own country, but rather for him who loveth the whole world. The earth is but one country, and mankind its citizens." - Bahá'u'lláh
 
njxc500
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:44 am

The airspeed would have indicated fairly accurately during takeoff, until they started to climb. Then the change in pressure would have also changed the airspeed, with the altimeter being useless. Hope that helps.

By the way this is a failure you can simulate in x-plane.
 
76er
Posts: 501
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 2:46 am

Before the AeroPeru and Birginair accidents the Boeing QRH did not have any procedures concerning unreliable airspeed indication. After the lessons were learned from these two events recall items and an accompanying checklist were introduced to deal with these pitot static system related issues.
Note: the radio altimeter will only pop up below 2500 AGL, above that the indications will be blank.
On the AeroPeru flight indications would have been normal during takeoff roll, since the pitot tubes were working normally. From what I can remember, on the Birginair flight only the captain's pitot tube was blocked, while indications on the first officer's side were nominal.

This is from the top of my head, so I may be wrong. But that is how I remember these to cases.

On a side note, the AA accident near Cali also resulted in changes to the Boeing QRH regarding ground proximity warnings.


Up until these 3 accident the 757 safety record had been perfect.
 
Redbellyguppy
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Re: Aeroperu crash question for pilots

Sat Sep 09, 2017 3:41 am

You could get a rudimentary altimeter and vsi by depressurizing the cabin, and ground speed at least from the IRS'es
 
axmiha
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Re: Aeroperu crash question for pilots

Wed Sep 13, 2017 6:09 am

Wow, VF, that's my answer right there. Fantastic. Thank you so much, mystery solved for me, finally. That is....until I read more, because there are more answers below, let's see what else the guys are discussing, but I consider myself answered. Thanks a lot. Of course I would suggest a simple "velocimeter" or "speedometer" like in any old Volkswagen, you know, just in case, but I'm sure I would be beaten up by a gang of pilots!
Best regards,
alex.
 
axmiha
Topic Author
Posts: 6
Joined: Tue Jun 13, 2017 4:02 am

Re: Aeroperu crash question for pilots

Wed Sep 13, 2017 6:22 am

Fantastic answers, thank you all. And VF already solved the mystery for me. Now the only mystery remaining is...how in the name of Christ maintenance forgets to take out the silver tape (!) and no one notices it? But I don't want to extend this subject too much. Thank you gentlemen. And njxc500, I wouldn't want to simulate this in X-Plane. I'd piss in my pants!

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