Fuel Tank Temperature

[Grahame Hutchison]

I saw this for the first time yesterday on OJB and then again today on OJE. Is this something new since the BA 777 fuel problem, or just the atmospheric conditions at present. Strange to have not seen it before now.

ACARS mode: 2 Aircraft reg: .VH-OJE
Message label: H1 Block id: 1 Msg. no: D41A
Flight id: QF0006
Message content:-
#2UBFTE 1
VH-OJE QFA0006
2109 29SEP08 WSSS YSSY DC 763 148 148
FUEL TANK TEMP BELOW -15 DEG C FOR 2.71 HRS
COUNT 6 PERIOD 1
GMT FT PALT LAT LONG SAT TAT MAC DEN
1826 -16 37001
-------------------------------------[30/09/2008 07:11]

[Will T]

Hi Grahame. Interesting you mention this - I saw this acars message in a thread on Pprune Tech Log about the same issue, albeit sent from a BA aeroplane.

It must be a new automatic downlink (definitely not sent by the pilots), and would almost certainly be related to the BA 777 incident.

[Mark D]

Originally Posted by Will Tidmarsh

Hi Grahame. Interesting you mention this - I saw this acars message in a thread on Pprune Tech Log about the same issue, albeit sent from a BA aeroplane.

It must be a new automatic downlink (definitely not sent by the pilots), and would almost certainly be related to the BA 777 incident.

Don't think it does, QFA monitor forecast OAT (outside air temp) and with the Jo'burg YSSY flights sometimes amend the route 'cos it's too damn cold. Dunno when kero freezes but they can't spend too long too far south.

[Will T]

That's correct, Speedy, we regularly run up against fuel temperature limits on the SYD-JNB route, depending on just how far south we fly. The B744 operations manual requires us to maintain a fuel temperature at least 3 degrees above the applicable fuel freeze point, and at -37C (3C above the -40C freeze point of Jet-A), an EICAS advisory 'FUEL TEMP LOW' is displayed.

Most of the ports we fly into refuel us with Jet-A1, which contains an icing inhibitor and has a lower freeze point of -47C. As such, provided the aeroplane has refuelled with Jet-A1, we can disregard the EICAS advisory and allow the fuel temperature to further decrease to -44C. However, the USA almost exclusively uses Jet-A (-40C freeze), and so if the aeroplane has refuelled in the US within the last 3 sectors, we need to consider all of our fuel as Jet A, and respect its higher freeze point.

The primary determinant of our fuel temperature is the TAT (total air temperature), which includes the friction heating effect of the airflow over the aeroplane. The fuel temperature will converge with the TAT over time, generally at a rate of 3deg/hr (up to 12 deg in 'extreme' conditions). As such, to manage our fuel temperature requires us to manage our TAT. We can increase the TAT by:

- climbing or descending into a warmer air mass
- re-routing to a warmer air mass
- increasing our Mach number (each M0.01 increase = ~0.5-0.7C TAT increase)

Obviously all well and good when fuel is surplus, but on long, limiting sectors such as SYD-JNB, there is little leeway in which to descend into warmer air (and away from optimum altitudes), or to re-route.

Another element on those long, southerly flights is the fact that the OAT can approach the limit of the aeroplane's 'environmental envelope' (around -70C at cruise altitudes), also requiring a descent or re-routing to warmer air.

As for the message described by Grahame, we don't send them from the flight deck.

[NickN]

Will your a goldmine for information. I learnt a whole heap of new info today from your post, thanks!

[Chris Roope]

Taken toward the end of SIN-LHR on a northern route.

[Montague S]

Originally Posted by Will Tidmarsh

That's correct, Speedy, we regularly run up against fuel temperature limits on the SYD-JNB route, depending on just how far south we fly. The B744 operations manual requires us to maintain a fuel temperature at least 3 degrees above the applicable fuel freeze point, and at -37C (3C above the -40C freeze point of Jet-A), an EICAS advisory 'FUEL TEMP LOW' is displayed.

thanks for the info, will this be a factor for the SYD-EZE sector starting next month? how far sound would that flight track?

[Will T]

Thanks guys. Chris's photo shows everything there nicely, including:

- the EICAS message (based on Jet-A temperatures)
- the TAT (displayed in top left corner) equal to the fuel temperature (bottom right corner, coloured amber due to the temperature)
- a relatively low SAT (Static Air Temperature, equivalent to OAT, or just plain ol' air temperature, excluding any friction effects)

Mont, yeah it'll certainly be a factor on those flights, although probably more so on the EZE-SYD sector, where flights would traditionally be routed into the deep south latitudes to avoid the strong westerly jetstreams around 30-40 degrees south. Conversely, eastbound flights will often take advantage of these jetstreams and so take a fairly northerly route. On more than one occasion I've flown JNB-SYD along the rhumb line track (basically a steady course along 30 deg S), which took us over PER and ADL, and entailed a steady tailwind in the range 100-150kts the whole time!

Our Flight Dispatchers monitor predicted OATs for all JNB (and EZE) flights, and where the aeroplane's environmental envelope or fuel temp limits are likely to be exceeded, will usually provide for a more northerly routing. These flights are now also monitored for solar radiation, and re-routed during a major solar event.