HF keying when refuelling represents fire hazard?

[noox89]

Hello,
FCOM Vol. 1 for 737-800 says: Do not keay the HF radio while the airplane is being fueled. Injury to personnel or fire can occur.

I suppose that this is related to HF radio waves, but why is the HF more dangerous than VFH radio waves? What are the odds that the airplane would catch on fire?

[KBPsen]

VHF radios typically have transmitting power of 20 watts whereas HF radios have up to 400 watts.

What the odds are of it causing a fire I don't know, but even if the probability is low the consequences would be serious. So the risk is high enough to warrant a warning.

[hetfield]

It's not the frequency (HF or VHF), it has to do with possible sparks at the antenna-coupler, in old days an electro-mechanical device.

AFAIK in modern planes fully electronic, so no restriction on refueling.

[noox89]

Thank you for the reply! I am just curious if this is the same as with: switch off the mobile phone prior to refueling. Billions of people refuel each day their cars and I have never heard of an explosion so far.

How can radio wave cause a spark? Can anyone clarify it physically?

[Checkboard]

In 1888 physicist Heinrich Hertz set out to scientifically verify Maxwell's predictions. Hertz used a tuned spark gap transmitter and a tuned spark gap detector (consisting of a loop of wire connected to a small spark gap) located a few meters away. In a series of UHF experiments, Hertz verified that electromagnetic waves were being produced by the transmitter. When the transmitter sparked, small sparks also appeared across the receiver's spark gap, which could be seen under a microscope.
Spark-gap transmitter - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Spark-gap_transmitter)

A gap which is the correct length in relation to the power of the signal and the wavelength can produce a spark across the gap. It's technically possible, but as you said, hasn't been spotted in the wild yet.

[Pub User]

As KBPsen mentioned above, the HF radio can be putting a lot of power into the antenna. With the aircraft on the ground it is possible that this power may seek a path to earth by jumping across little gaps and creating sparks.

Low probability of a catastrophe, is it worth the risk?

[Avtrician]

The HF antenna in the old long wire systems could cause a spark if the conductor broke, but the modern Leading edge type are most unlikely to have this problem. The damage to persons is the more likely, as HF can cause a nasty burn if you touch the antenna in the wrong spot.

If you think about the conditions needed to start the engines, ie air flow, atomised fuel and a very high energy igniter, the the likley hood of having/causing a fire is extremely remote. try an experiment at home and see how hard it is to get kero to burn with a match.

[Type1106]

I think a lot of these precautions go back to the days of piston engine ac using high octane gasoline. As has been said, it's difficult to get kerosene to burn under these circumstances BUT kerosene in vapour form is a different matter if the temp and air mix is right.

I seem to remember that the USAF operated a turboprop back in the 60s called the C133 which had a habit of disappearing on overwater flights. Wreckage indicated an explosion in an empty tank and I believe it was concluded that the explosion was related to the use of the HF. Maybe someone has more info?

[aviatorhi]

If memory serves correctly I believe that two C133s lost over the Atlantic were lost about a year apart, over the exact same reporting point. :eek:

I was, however, always under the impression that they had a nasty high altitude flight dynamic which would quickly overcome the abilities of someone ill prepared for high altitude flight at the limits of the envelope.

[Phalconphixer]

Just hold a 4ft fluorescent tube close to a transmitting HF antenna for a demonstration of the amount of power being produced.

Also worth remembering that in the case of a notch antennae the whole airframe is effectively the radiation source... and you really don't want to be transferring fuel when transmitting.

[custardpsc]

I can't remember the source but I did read that the significant danger with mobile phones is dropping them while refueling and the battery pack separating/breaking. If it is on there is a risk of a spark. This was a while ago when battery packs were often easily removable but the main point was a spark occuring near the ground where the dense flammable vapours would pool.

Keying any transmitter has a theoretical risk of a spark in a loose antenna connection. Tenuous - sure. Possible - yes.

[custardpsc]

Found the source re phones and fueling - here on PPrune ! - a good read

Refuelling and Cell-Phone Use [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-145851.html)

[763 jock]

We used to have to test our HF radios prior to ETOPS flights. This was almost 20 years ago.

All it ever proved was that it worked when you tested it. It didn't prove that it would still be working 5 minutes later. :confused:

[lomapaseo]

I can't remember the source but I did read that the significant danger with mobile phones is dropping them while refueling and the battery pack separating/breaking. If it is on there is a risk of a spark. This was a while ago when battery packs were often easily removable but the main point was a spark occuring near the ground where the dense flammable vapours would pool.



best not to drop your flashlight or I-pod as well

Best to opt for the full service option and let the attendant off himself instead

[awblain]

RF fields near antennas can be more intense than the radiation field at large distances would suggest, if scaled back by the inverse square law.

Thin conductors nearby can be heated by induced RF currents, and it's also perhaps not impossible for them to cause charge to jump a tiny gap and spark.

Aircraft radios can radiate hundreds of watts, whereas mobile phones produce about one watt. A fire from a mobile phone on a garage forecourt is likely to be urban myth, but I'd say that there is a more realistic possibility that an aircraft radio could start a fire.

[fire wall]

Also when dumping fuel as fuel is atomized and, on some a/c (747 classic is but one) the HF antennae (sp) is in close proximity to the fuel dump nozzle.

[fireflybob]

We used to have to test our HF radios prior to ETOPS flights. This was almost 20 years ago.

All it ever proved was that it worked when you tested it. It didn't prove that it would still be working 5 minutes later

Fair comment but it's nice to know the SELCAL works before you despatch - ever had to listen out on HF for a few hours?!!

[Mike744]

When loading aviation fuel on marine tankers we were required to ground all transmitting antennas (MF & HF) up to 2KW however I do not recall any limitations to the use of the ships VHF transmitter (25W). There was however prohibition on using hand-held units on deck (VHF & UHF) as they were not classified as intrinsically safe under loading conditions. There would have been a considerably greater venting of vapour here than fueling an aircraft however because of loading at 10,000 tph.

[Dan Winterland]

Also bear in mind that the HF aerial is sometimes the whole aircraft itself. Although the tech manual may give the location of the HF aerial as being the fin, or some other part of the aircraft, this may just be the ATU (Aerial Tuning Unit) which matches the airframe to the frequency being used.

[Machinbird]

Short sea story:
We had two F-4s parked on the bow of USS America CVA-66, and they were from different squadrons. Next morning, the fuel quantities on both aircraft were spinning.

Trouble shooting revealed that one of the fuselage fuel probes in the same location on both aircraft was dangling from its wires inside the tank. These probes were held in their position by snap rings in a groove.

What does that do with HF? Just that carriers have long dipole transmitting antennas spaced along the sides of the bow of the ship. For flight ops the dipoles are horizontal, but when the ship is not flying, they can raise them to vertical.

Did RF couple up to the fuel quantity wiring and cause an explosion inside the probes? I don't know, but we certainly had our suspicions.:suspect: I never saw that particular malfunction again.

[ChristiaanJ]

Machinbird,
I'd say that is how urban legends are created ....
A fuel tank is still to a large extent a 'Faraday cage', so RF would not have such a major effect.

"I never saw that particular malfunction again. ".
Your remark, Occam's razor, Murphy's law, etc. would say, AFAIAK, that a causal link between the fuel system failure and the HF system does not exist....

Personally, I would think a link between fuel fire risks and RF is more or less a red herring.
Sparks and fuel, yes. RF and fuel, no.
I'm interested, which is why I 'subscribed' to this thread.
But so far, I've not seen any 'technologically sound' discussion.....

CJ

[Machinbird]

I'd say that is how urban legends are created ....
A fuel tank is still to a large extent a 'Faraday cage', so RF would not have such a major effect.
ChristiaanJ, It was an extremely unusual and still unexplained event. I actually held one of the probes in as-removed condition in my hands. Having a probe come loose from its base is extremely unusual. That snap ring should have been good for the life of the aircraft. Having two on the same type aircraft on the same night, which were parked in close proximity is even stranger, particularly when you consider that the fuel indication systems were working OK on shutdown. These aircraft had been refueled immediately after landing. Unlike Avgas, JP-5 could form explosive mixtures with air depending on fuel temperature.

The presence of strong RF fields could sometimes be verified by the sparks playing on the links of the tiedown chains at night.

You would think that the skin of the aircraft would form an effective Farraday cage, but if the length of a panel is similar to a wavelength being transmitted, some coupling could occur. These panels were bedded in a thin layer of conformal sealant that tended to isolate them from the underlying structure. Immediately below were wire bundles, sensors, control cables and things like that. Without doing actual measurements, I'd say, "Never say never."

[ChristiaanJ]

Machinbird,
Thanks.
I totally agree with your "never say never", especially since "you were there".
But I'm still baffled........

CJ

[Mike744]

There is an IEEE paper on this subject available to members:
"An investigation was performed to study the potential for RF power radiated from portable electronic devices (PEDs) to create an arcing/sparking event within the fuel tank of a large transport aircraft. This paper describes the experimental methods used for measuring RF coupling to the fuel tank and fuel quantity indication system (FQIS) wiring from PED sources located in the passenger cabin. To allow comparison of voltage/current data obtained in a laboratory chamber FQIS installation to an actual aircraft FQIS installation, aircraft fuel tank RF reverberation characteristics were also measured. Results from the measurements, along with a survey of threats from typical intentional transmitting PEDs are presented. The resulting worst-case power coupled onto fuel tank FQIS wiring is derived. The same approach can be applied to measure RF coupling into various other aircraft systems" ISBN 0-7803-6395-7

[iwrbf]

Hi,

depending on the configuration of the transmitter <-> antenna construction, there are at least two hazards (as written above, just for clarification once more...)

1. Transmitted RF has a resonance frequency on a resonance wavelength.
(And some multipliers above and beyond that, their strength depends on the quality of the used gear, mainly...)

This transmitted RF is usually much higher when HF is used than in VHF/UHF applications. This has to do with the benefits of higher wattage in HF (the gain in the higher frequencies of VHF/UHF is not as large as in HF's frequency regions).

The transmitted RF does include very high voltages and very high currents. (Almost) never at the same place but ... see on (2) below :-)

RF will bring everything in (electrical and/or magnetic) resonance that fits its needs. If an HF radio transmits - for example - in the 20m wavelength region, there'll be resonance on every metallic object that is 5m long or a multiplier of this (this is a cruelly simplified explanation). There can be even resonance on shorter or longer objects due to special effects like electrical or magnetic shortening or increase in e. or m. length due to capacities etc.

The argument with the Faraday Cage is not valid when the RF is emitted by the refuelled object. It's thinkable that an HF transmitter sends on a fixed frequency and the wing which has a fixed resonance frequency is 'adjusted' by the fuel level (think of a fuel tank as a large capacitor with a very large distance between platters and then fill the capacitor with Jet A1... ;-) )

There are countless possibilities of resonating airframe, malfunctioning fuel pumps, high induction currents on the pumps' motors etc. - all created by a simple coincidence in resonance between your HF radio's frequency and ANY object on the airplane that resonates on this frequency.

2. There's no ideal antenna.

First, antennas are always imperfect. There's no practical antenna solution that transmits all of the HF energy as resonating HF. There's always some electricity 'left' due to a wandering standing wave. (SWR Standing Wave Ratio)

This electricity usually works as an antenna heater ;-)

If there's the slightest problem with the antenna, this current can become really significant (almost every HF problem is due to antenna related problems, aerial RF that is...) - imagine this current finding a way from the HF antenna to your fuel, finely vaporized whilst refilling your tanks with high pressure... all you need is ONE broken insulation or a 'high quality Seattle soldering' out of place... ;-)

3. Jet A1 is not Halon.

One has to differentiate between a puddle of Kerosene on the ground and finely vaporized K. in a fuel tank. The latter is by far more prone to a spontaneous adventure in exothermous reaction... ;-)

What I want to say:

Please don't be mindless. There may be only a slight possibility of an incident regarding HF with Kerosene or other fuels... BUT... it's really possible.

Personally, I would not jeopardize my career, my plane or the lives of the people involved for a simple 'I don't believe in that'.

Kind regards,
Peter

PS: Even if the fuel has no problem with your el cheapo HF with its crappy antenna on your multi million dollar airplane. Seeing your fuel specialist falling down his ladder with severe HF burns on his fingers won't make your day, believe me. These burns are nasty and painful as hell and they tend to take a long time to heal, similar to fluoric acid. Don't play at work!

[bubbers44]

Our airline prohibited us from HF transmissions while dumping for a very good reason. About the same reason you don't smoke a cigarette while fueling your car. You might get by with it your whole life and you might not.

[EEngr]

The primary problem with HF transmission while refueling is the possibility of coupling a significant amount of energy (enough to produce a spark across a small gap) in some nearby metallic object.

The aircraft, antenna, tanks and wiring may be in perfect shape. All failure modes of these subsystems may have been accounted for and designed to prevent inadvertent ignition of fuel vapors. But the situation on the ground is uncontrollable. There is no easy way to test every object that may be present within the near field of the HF antenna while on the ground for its ability to couple some radiated energy during a transmission. And since the area around fuel dispensing operations is classified as hazardous, eliminating possible ignition sources may require the prevention of introducing RF and/or electrical energy into them.

[grounded27]

I thought it an issue with classic 747's as the couplers were out in the wing tip. (during fueling) seems they expect the possibility of leaking/exposed fuel. Can anyone document a fire caused by an HF key during fueling in the last 20-30 OK even 40 years?

[spannersatcx]

I thought it an issue with classic 747's as the couplers were out in the wing tip. (during fueling) seems they expect the possibility of leaking/exposed fuel. Can anyone document a fire caused by an HF key during fueling in the last 20-30 OK even 40 years?

As you are not supposed to do it during refuelling then there should be no such incident! :eek:

[grounded27]

As you are not supposed to do it during refuelling then there should be no such incident!

My expectation was that this was a policy written based on experience.

[EEngr]

My expectation was that this was a policy written based on experience.
It may be due to some analysis as well. Having done a few failure modes and effects analysis in my time, its common for engineers to identify such modes, perhaps do a bit of testing and revise a design or establish operational guidelines to prevent an actual situation from arising. The ability of an HF transmission to cause a spark in a nearby structure may have been something they observed in a lab.

Sometimes we don't wait for things to blow up before we fix them.:8

[grounded27]

It may be due to some analysis as well. Having done a few failure modes and effects analysis in my time, its common for engineers to identify such modes, perhaps do a bit of testing and revise a design or establish operational guidelines to prevent an actual situation from arising. The ability of an HF transmission to cause a spark in a nearby structure may have been something they observed in a lab.

Sometimes we don't wait for things to blow up before we fix them

Wonderful theory.

Q: How old are aircraft? How old is HF as used in aircraft and what was the wattage of the first systems?

Now like I said before I could understand this in a 742 with the couplers and ant in the wingtips but the same policy is followed on the MD-11 with couplers and ant in the tail under the #2 inlet... There is no nearby structure that a spark could ignite fuel from. I claim wives tail based on an experience of a Darwin award recipient many years ago.

[Piltdown Man]

Just for the hell of it, look at the ammeter the time you hit the PTT on the HF. From memory, the ones I used took something like 30-40 amps. Quite where all that powers goes I'm not sure, but not all of it stays inside, so to speak.

PM

[ChristiaanJ]

IGh,

The two examples you mention both were ESD, not RF effects.

As to the autothrottle certification problem you quoted... I suppose you've seen my 'tale' about Concorde, where during early flight tests the a/c 'wagged its tail' during HF transmissions.
On Concorde, the HF aerials were two large slots in the vertical fin (visible on photos of the British 002 prototype, because there they were not painted for some reason) and they set up enough HF currents in the tail structure to interfere with the signals from the autostab yaw rate gyros, which were also fitted in the tail. Cured with some simple filters in the signal wires.

Apologies for the slight TD.

CJ

[grounded27]

-- UA562 / 3May70 Boeing 727-100, N7027 UA ship # 7527, center fuel tank "exploded" during refueling. Parked on ramp, MSP, then number Gate 24. Wx: 53 deg F, wind 290/ 8. Fueling had been in progress for eight minutes when a LOUD sharp BANG sounded (like a fire cracker); fuel vapor and smoke seen ejected from Left Wing Tip near Tank Vent and fuel dump outlet [witnessed from outside by S/O, Oz mechanic, and the fueller]. Fueling was stopped; S/O disconnected electrical power from the buses, APU then was shutdown. Inspection: explosion in #2 Tank (center) ... presumed static discharge within #2 tank induced by presence of steel clamps and bolt assembly . . . Investigation revealed that fuellers at MSP used 450 gpm rate ... F/A's inside cabin were asked about the sound (explosion), but they perceived only a bump as if someone had hit the a/c with a belt-loader. ...


I had heard stories from old 727 techs of hearing a tank explosion (low fuel level, ACM's running, high OAT), they said they heard a loud THUMP. FLT 800, EWIS was driven by this allthough with many stories told about tank explosions (with no breach) I do not believe it was the cause of flt 800. None the less it had never been linked to HF key. Our now standard wiring practices are a good thing.

[RR_NDB]

Hi,

noox89: (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard.html#post7094734)

Will elaborate asap on interesting issue. For now:

1) Several factors to consider when estimating probabilities: Fuel type, Antenna type, Power output (maximum at LOUD voice), Aircraft type, Aircraft condition, Refueling equipt. and condition, Temp, Dew point, etc.
2) In general the probability to fuel ignition i consider very low (Jet A-1).
3) On mobile or HT's (VHF/UHF) their batteries could be dangerous. (e.g. LiPo). Not the RF due it's lower out power.

I operated HF (high power) mobile (land) since 1970 in practically all possible situations and configurations (testing different antennas) and would say: I would avoid to key PTT during (ground :)) refueling. Only if necessary. But IMO the risk of fire (in general) is low.

I will comment (tech. aspects involved) like "parasitic resonance" of nearby structures.

The FCOM mentioned i understand as a standard to simply put chances at zero.

HF keying when refuelling represents fire hazard? (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard.html#post7094734)

Interesting question and will have pleasure to comment the many aspects involved.

Mac

PS

Machinbird:

Did RF couple up to the fuel quantity wiring... (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard.html#post7098968)

Would like to study a little bit on this case. Position of A/C in respect to nearby HF poles, HF power output (Certainly > 1KW), location of the probes, etc.

The presence of strong RF fields could sometimes be verified by the sparks playing on the links of the tiedown chains at night.


Structures may resonate and develop High Voltages (and flash)

PS2

On RF issues i remember the EA6B canopy (gold shielded) because the jamming antenna behind. :E

PS3

There is A LOT of myths in this issue. Even among EE. This is an "analog" field and the feeling of the technicians are decaying due the "digital age". Few people really has expertise on the issue. The comment of Avtrician (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard.html#post7094931) certainly comes from an expert.

PS4

Type 1106: Maybe someone has more info? (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard.html#post7095630) This Strategic transport used "long wire". Let's look.

PS5

Don't play at work! (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard-2.html#post7099265)

:ok:

PS6

bubbers44 (http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard-2.html#post7099467)

On "dumping", (jetisoning) a TACAMO (with it's 30,000 ft trailing wire) fed by the 5,000 ft "exciter" should not, indeed. :} (200,000 Watts RF power :eek:)

[RR_NDB]

Hi,

grounded27:

How old is HF as used in aircraft and what was the wattage of the first systems?
(http://www.pprune.org/tech-log/480601-hf-keying-when-refuelling-represents-fire-hazard-2.html#post7100898)
Amelia Earhart tried HF use (unsuccessfully) in her doomed flight in 1937. They were heard (IIRC, loud and clear) on Itasca (ship) but a probable aerial broked in Lae take off reduced drastically the ERP (effective radiated power) only allowing short distance comm.

I will study to trace the beginning of use of HF on A/C.

The capabilities of first Systems were very limited. They used CW (morse code) thus compensating equipt. limitations. Later on they introduced AM (phone) with limited performance due the use of AM (amplitude modulation. The same used currently used in VHF comm. today).

The power levels were low (due derated RF tubes to increase "TBO" MTBF). For example, the "powerful" Collins ART13 (extensively used in WW II) was rated 100 W only. I have a Bendix TA12 (a good equipt.) with just 40 Watts (AM) from WW II. The "Command Set" designed for gliders. used same power level.

With the introduction of SSB (an highly improved AM) the power levels went to the 400 w PEP (peak envelope power) standard today. Enough to you be heard by good operators in ground stations. (not always the case :})

Based in my experience of ~ 5,000 hours of (land mobile) continuous HF use (SSB or CW/morse 500 W power level) i can say with a proper selection of frequency and with a good (efficient) antenna you have (most* of time) WORLD WIDE comm.

The problems faced in A/C using HF are to be commented later. Derives from several factors.

Mac

(*) With CW (Morse) ALL TIME: 99,99%, :)

[grounded27]

Thank you for the good read RR_NDB! Google has failed me on this search. Actual events of danger during ground refueling still are not to be found but I much appreciate your post!

[RR_NDB]

Hi,

MB:

.. but if the length of a panel is similar to a wavelength being transmitted, some coupling could occur.



Was not the case because suspect would be HF through pole antennas. Specially if put vertical (few feet from A/C) and delivering high ERP (effective radiated power).

Do you remember on the use of frequencies near high end of HF spectrum? (30 MHz)

Another point. Certainly would not be caused by SSB or CW. Should be data. (duty cycle considerations-time duration of high power use)

Question: The probes suffered? Had to be replaced? Do you remember?

Mac

PS

The plane could act as a "path" to conduct RF amps (as a ground plane) above carrier deck.

Anyway, i consider unlikely RF caused the issue.

But i still don't discard the possibility.

PS

Which other reason could explain the facts (in TWO planes) Any other cause?

Or other possibilities should be discarded?