peebs24

It is highly unlikely that any amount of RF energy (within reason) would actually move a mechanical device like a relay. What might happen is that the rf field might effect the electronics that drive the relay but any such circuit that was safety concious would normally be shielded.

peebs24

Quote

To follow up on Chris Scott's query and ionagh's reply, here are a few links from twenty years ago to problems the US Army may have had with EMI affecting its Black Hawk helicopters (I emphasise the words "may have had", in contrast to "did have"!). The Navy apparently did not have similar concerns with its Sea Hawks: they were better shielded because of the extremely EM-loud shipboard environment. endquote

Many years ago flying Canberras we had a situation where hitting the push to transmit button energised the explosive bolts in the canopy blowing it off. The VHF antennas were moulded into the perspex canopy and the feed to the antennas ran parallel to the explosive bolt wiring. If the shielding on the antenna feed broke the entire output of the vhf transmitters was coupled straight into the explosive bolt circuit and generated sufficient current to ignite the bolts.
This was considerable amount of rf energy in very close contact with very sensitive circuits and I doubt that in modern aircraft rf energy sources of this magnitude are anywhere near the fuel control system.

shawk

Probably off topic and very unlikely, shrug.
Unusual acoustical standing waves in piping can restrict fuel flow. These are very difficult to recreate or analyze, as they leave little evidence. These standing waves generally depend on a combination of fluid flow, pump speed, pipe geometry, external vibration and fluid density.

Also, the investigation board mentioned cavitation damage to the fuel pumps.
In your collective opinion, how long would it take for cavitation to damage a 777 fuel pump? Could the damage could happen in the time preceding this accident or would it take longer?

lomapaseo

would you care to restate the question (in bold) again

Chris Scott

Hi lomapaseo,

Your Ameican colleague shawk has logged off. Think what he/she may be asking is:

Do we infer that the HP fuel-pump cavitation damage - as found by the investigators - all took place on the accident flight? Or had it been building up over many flight-cycles, i.e., had there been undiagnosed fuel starvation on previous flights?

Good night, Chris

suninmyeyes

For those who are looking for a cause of the accident to be the fuel pumps. I just wanted to say the 777 engines will still run quite happily on suction feed with all fuel pumps switched off provided you are not at a high FL.

In fact if you have to do a partial gear landing the checklist instructs you to switch off all fuel pumps on approach.

Chris Scott

It was the Engine HP fuel pumps (mechanical, from an accessory gearbox, and unswitchable) that were cavitated, not the (LP) Tank pumps...

Machaca

Christopher Brennen's books
are available online in their entirety courtesy of the author and CalTech.


Have at it!



-=MachacA=-

bill_s

Some basics might help. An electromechanical relay is a coil of wire around a magnetic (iron) core, and a magnetic armature that moves one or more electrical contacts when the coil is energized.

A typical 28 volt DC relay needs at least 18 to 20 volts DC applied to its coil in order to operate. There is no way any radio frequency signal short of a lightning hit will operate or release the relay directly.

Here's the fine print part: relays often have a diode wired across their coils for the purpose of suppressing the high voltage pulse that the coil generates when you suddenly de-energize it. Depending on the speed of this diode, and the topology of related circuits, the diode could turn RF energy induced into the wiring to the coil into enough DC to operate the relay, or drop it out if it is already energized.

The problem is that you would have to induce some 20 volts of stray RF directly into the relay coil wiring, for a significant period of time, for this scenario to work. This RF would have to penetrate both the acft metal hull and any shielding on the wires. I have no idea how much RF power would have to be delivered to the outside of the hull, but my guess is it would be upwards of tens of kilowatts. This massive amount of RF power would probably disrupt other electronics in the acft long before it acted directly on any relay.

(DC = direct current, RF = radio frequency energy)

Halfnut

http://www.alliedpilots.org/Public/P...ne/hotline.asp

This is APA Communications Director Gregg Overman with the APA Information Hotline for Thursday, February 28.

777 SAFETY ALERT: This afternoon American Airlines Flight 229, a Boeing 777 bound from MIA to LAX, had the left engine hang up on approach to LAX at approximately 2,000 feet. The auto throttles were on and the left engine hung at approach idle as the right engine accelerated normally. It is believed that the left engine would not respond to throttle inputs for 10-15 seconds before finally responding and accelerating to the commanded thrust. The right engine performed normally. Maintenance has downloaded the DFDR data and Maintenance action to be taken (ATBT) will be to sump the fuel tanks, pull the engine fuel filters and check for contamination, BITE check the EEC and check the MAT for any existing faults. We will continue to update you as we get more information.

ve3id

I think you are barking up the wrong tree here! I just did a quick calculation using 28 Volts across a 1k ohm load (the coil at RF) and used 140dB for space loss beyond the e-field.

I came up with 78 TeraWatts!

If someone was using that kind of RF power, you would know. All the lights in and around the airport would dim!

GlueBall

Halfnut you don't specify engine model; is AA using Rollers?

hellsbrink

I've seen RR engines on some AA 777's, Glueball

Graybeard

Will the authorities ground the 777 fleet now? They did it to the worldwide DC-10 fleet in 1979, wrongly as it turned out.

GB

Milt

Another Acceleration Hangup on a B777.

This may be a blessing in disguise!!

Remarkable and let's hope 'they' can identify the cause. Presumably the fuel system and FADEC is standard to whatever engine leaving only the engine driven High Pressure fuel pump/s as a variation.

What doesn't seem to have been covered so far on this thread are the ACUs - Acceleration Control Units. Are these part of the FADEC and hence computer controlled and are there variable compressor guide vanes and compressor bleeds in the engines also being controlled by the ACUs?

Remember the RR Avons and the gubbonry necessary to have them accelerate cleanly what with guide vanes and bleed valves inter related. Was it 18 or 28 seconds from idle to max following a throttle slam? Once had one of these in an F-86 Sabre maladjusted under investigation hicoughing around a circuit with some of the surges putting out the fires. Saved from deadsticking with easy HOT relights.

For the uninitiated a HOT Relight is defined as one that can often reliably be achieved within 10 seconds of a flame out.

NSEU

I can confirm Jetdoc's(?) statement that the (2) surge tank pressure relief valves have both positive and negative pressure relief. There is a description in the AMM on how to check the valve using positive and negative pressures. So the vacuum in tank theory seems to have lost its momentum

On being sanctimonious...
My company does not allow me to take information from company computers without permission.
All the Boeing manuals on my company computers have very prominent copyright statements.
No doubt similar restrictions are placed on those posting pictures here.

Also...
Some of the technology used on the 777 is classified material. Boeing recently got into trouble for selling airplanes to a foreign country.

What copyright de-restrictions are placed on Americans are not necessarily the same as those placed on other nationalities.

There is a difference between being sanctimonious and worrying about fellow PPRuNers losing their jobs and being punished financially and/or being imprisoned.

Rgds.
NSEU

NSEU

Note however...
The trigger point for fuel pump low pressure switches can be as low as 4psi... The output of the boost pumps is, nominally, 12psi... so the pumps can, in theory operate down to almost 1/3rd the normal pressure before a warning is triggered.

Also, flow rate can vary depending on the type of 777 boost pump ...even though the output psi is the same. One 777 airline AMM shows the data for 2 pumps. Flow rate for one is 16,000kg/hour. Another is 19,320 kg/hr.

I'd be interested to know what sort of pressures are required to open the check valves on the boost pumps.

Rgds.
NSEU

28L

Ref the AA 777 incident yesterday, I understand the problem was probably with the autothrottle rather than the engine. Entirely different to the BA 777 incident.
Hope this stops the thread diverging.....I'm learning a lot about cavitation and bubbles here!

sky9

Just a follow up of the AA :

http://www.flightglobal.com/articles...-throttle.html

woodpecker

Rather a strange statement, let me explain.

The auto throttle drives the throttles>>> the throttle position (angle) is sent to the Fadec (EEC)>>>the Fadec sends demands to the fuel metering valves to give the appropriate power.

For the problem to have been due to the autothrottle system it would have needed a failure that, when power was demanded, only moved one of the throttles forward (which could have simply have been rectified by moving the lagging throttle forward).