HF keying when refuelling represents fire hazard?
 

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?

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.

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.

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?

Spark-gap transmitter - Wikipedia, the free encyclopedia

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.

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?

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.

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?

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.

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.

mobile phones refueling
 

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.

Source of above comment
 

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

Refuelling and Cell-Phone Use [Archive] - PPRuNe Forums

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:

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

Induced currents
 

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.

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.

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?!!

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.

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.

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.