Fuel Decision in Military Aviation [Archive]

Engines

23rd Mar 2016, 07:48

LW20,

Perhaps I can help a bit.

In my experience, most land based jet fixed wing sorties are fully fuelled, at least the internals. With external tanks, again usually fully fuelled.

On carriers, this is definitely not the case, especially if weapons are being carried. For catapult launch fuel load is often juggled against other factors (e.g. Wind over deck, sea state, individual aircraft weight, stores carried, catapult condition, sortie profile) to allow a safe launch from the ship.

For STOVL ops, it was usually full fuel, given the fairly limited fuel loads available on the first generation (Harrier) airframes. Howeve, this could be traded against weapon loads for short sorties.

For all carrier launches, there will also be detailed CG calculations to support application of correct trim and propulsion system settings.

Complicated stuff, this naval aviation.

Best regards as ever to all those doing the sums out there on the ocean deep,

Engines

hunterboy

23rd Mar 2016, 09:11

I'm guessing you could launch off the carrier with full weapons and min fuel straight to the tanker? Makes you wonder why they don't make the catapult longer 😚

Evalu8ter

23rd Mar 2016, 09:22

HB,
My good friend Engines will doubtless correct me here, but the energy provided by the cat stroke would not be improved by making the run longer; in fact, in might start to slow? Your assumption about lifting light on fuel and AAR is a valid one, and used by several aircraft to achieve max ordnance at take off, whilst still retaining a margin against, say, the loss of an engine. Unfortunately, Naval AAR is somewhat lacking at the moment (and, save possible "buddy-buddy") not an option of the RN/RAF F-35 Sqns when embarked, unless, of course, they operate within range of land based AAR.

Helicopters are forever making fuel/payload/range trades - exacerbated if you are operating at high Density Altitude. For example, in the CH47 you'd normally fly full internals in the UK, whereas in Afghan you'd typically refuel to approx 2/3rds to give you some planning margin (as, with the exception of the Mk3, the CH47 doesn't have a fuel dump). If you are lifting an ultra-heavy load you would plan to burn down to the min required fuel. This was always a challenge in the FI as you could often be tasked to lift heavy items some way away from the nearest fuel.....

TorqueOfTheDevil

23rd Mar 2016, 09:26

In the SAR world, there's a bit of compromising between performance and endurance to be done. For instance, if you are going to rescue someone from a mountain, you don't want much fuel on board, but if the position is uncertain, you need enough fuel to find them first. If the task is a search, you need to establish (based on weather and availability of other assets) whether you want to arrive with plenty of fuel to give you a worthwhile amount of time to search, or arrive with not much fuel to allow you to deliver MRT quickly onto the hillside (takes a long time if you can only move 3 or 4 at a time!). Throughout all of this, one has to remain aware of distance to the nearest fuel site, what time that fuel site is open, how easy it will be to get there based on day/night/weather etc etc.

In the RAF SAR Force, we used to hedge our bets somewhat and have the aircraft approximately half full - 3000lbs of fuel would give nearly 3 hours endurance (enough to get to the job and arrive with enough performance to winch, get the casualty to hospital and find a refuel site in most cases). There was always a bowser on hand to top up the aircraft if a longer-range job (or search) came in. If one wanted to take full fuel, the aircraft needed to be stripped due to the weight of the role equipment. Inevitably, this took a bit of time but a lot of the stuff could be removed fairly quickly - it was only taking the seats out etc which took a bit longer, and on a proper long-range job the crew would take time to check the fuel plan before launching so the engineers had time to strip stuff out anyway.

At least one unit used to reduce the standard fuel load in the summer because the mountains were on the doorstep, whereas other bases were further from the hills so would burn more fuel getting there. If one arrived on scene without sufficient performance, there was always the option to chuck fuel over the side, but it's not a popular move given how many SAROps take place in National Parks...I only saw this done once when a visiting Captain gassed up the aircraft to MAUM ready for a good long training sortie all over the Highlands in glorious summer sunshine, and we promptly got scrambled to two cragfast climbers who had made it almost to the top of Ben Nevis...cue dumping of fuel most of the way down the Great Glen, and more on scene when we got there and tried to hover and still ran out of power:{.

Jumping_Jack

23rd Mar 2016, 09:42

Lancman

23rd Mar 2016, 10:02

Going back a bit now, like 65 years, on a Shackleton it was 1,200 gallons for pounding the circuit, 2,200 gallons for a 6 hour training flight which might include radar homings, ship photography and bombing and gunnery, and full tanks (3,292 gallons) for more serious stuff. The Flight Engineer did a weight and balance sheet for all the longer flights.

Engines

23rd Mar 2016, 10:27

HB,

Evalu8ter is, as usual, bang on the money. There is a close link between USN catapult designs and USN aircraft, and longer cats would not deliver much improvement, as aircraft end speed is as good as it already can be.

Another factor is ship design - the CVN flight deck is essentially sized by the requirement to get 4 catapults installed. Longer cats would mean a new flight deck, and a longer ship, which would need more power, so an extra reactor.....

Launching off the cat at low fuel straight to a tanker usually happens when the aircraft design has gone wrong - the Buccaneer S1 was so badly underpowered it could only launch at low weights, with low fuel. Or it can happen if an exceptionally heavy weapon load is carried - a very rare event.

Hope this helps

Best regards as ever to those going off the cat

Engines

RetiredF4

23rd Mar 2016, 10:44

The reason in the civvie world for taking off with a precomputed fuel load, doing takeoffs from other positions than the beginning of the runway, and not using full available power is an economic one.

That said I can only talk for fast landbased jets, where the most useless things are the fuel left on ground and the runway behind the aircraft.

Concerning the fuel there were sorties, which called for less fuel due to tactical and structural reasons. It is no good to go into a WVR engagement with three full outboard tanks. Either those stayed at home, were not refueled, or were flown dry with sometimes sensless maneuvers. In the F4 we could refuel the jet without the fuselage tanks 5 and 6, which was used for a quick combat dart sortie over the north sea close to our airbase.

RedhillPhil

23rd Mar 2016, 10:51

I'm guessing you could launch off the carrier with full weapons and min fuel straight to the tanker? Makes you wonder why they don't make the catapult longer 😚

I believe that the Buccaneer S1s used to do that. Launch with weapons loaded and a light fuel load then go straight to a buddy fitted Scimitar tanker.

Tankertrashnav

23rd Mar 2016, 11:18

Calculating takeoff fuel was pretty crucial when operating in the tropics in the Victor K1 tanker. Max fuel load was 86k pounds, but this could be down to 55k when taking off from, say, Masirah, with temps in the high 40s.

We were once down to transit from Marham to Leuchars to position for an exercise, when our aircraft, which had 40k of fuel on board for the 40 minute flight, went u/s before takeoff. The only alternative aircraft available had 86k on board, and my impatient flight commander captain wouldn't wait for it to be defuelled. I spent most of the transit jettisonning around 40k through the pods to get the aircraft down to max landing weight. I can imagine how well that would go down in the civvy world!

LW20

23rd Mar 2016, 11:28

I see two different worlds. (We just discussed if to take 94 or 95 tons). Thanks for the quick replies.
More comments/ explanations are highly appreciated.

KenV

23rd Mar 2016, 13:38

one of our most imported decision in commercial aviation is about fuel to be loaded.
How is it handled in military aviation? E.g: Do fighters always top off? How is it handled on carriers? Etc,Tactical military aviation is very different than commercial airliner aviation with regards to fuel loads, and the takeoff fuel loads depend on many factors. However, in general as much fuel would be loaded as the situation permitted. (in other words, you loaded fuel to fill all your tanks, or you loaded fuel till you hit some operational constraint, like max take off weight, max catapult weight, sea state, etc.)

However, military transport and patrol aviation is more similar to commercial aviation. You compute the amount you need for the mission, plus reserves, and load that. In the US P-3 community we computed the amount needed for the mission, plus reserves, and then added 6 klbs "for the wife and kids".

The SR-71 never took off with max fuel. They ALWAYS took off with a light fuel load and then hit a tanker to top off. They essentially never went anywhere without tanker support.

The B-52 has a higher max inflight gross weight than max takeoff gross weight. So when taking off with a big payload requiring long range, they would load enough fuel to reach or nearly reach max takeoff weight and then hit a tanker to add sufficient fuel to reach their max inflight gross weight.

WIDN62

23rd Mar 2016, 14:03

In the C130 world at Lyneham it was routine to offload freight in favour of more fuel, just so you didn't need to stage at Gander when travelling west. I'm sure it was in the best interests of defence to fly fuel rather than cargo....
Jumping Jack,
I am not sure where you have got that from, but you are wrong - it was never routine! If some of the freight was a top priority and had a specific time frame, then it could be a possibility, but such occurrences were and are rare.
Also, flights are rarely scheduled further than Gander or Goose because of Crew Duty Time restrictions and most had the option of a flag stop at Keflavik to ensure that the maximum payload would be offered.

Pontius Navigator

23rd Mar 2016, 14:48

On the Vulcan the normal fuel uplift was full except when the calculated fuel burn on a short sortie would have given a landing weight above normal maximum. Typically air tests or displays would be fuelled to minimums for the sortie.

I suspect you are really looking for short leg comparison. For the short hop UK-Malta, once flown in under 2 hours, we still fuelled to max.

However I know an ex-RAF nav on Britannias with Britannia quit as all he did was fuel economics - minimum sector fuel uplift with expensive fuel - maximum cheap fuel uplift compensure with cost of carriage etc.

Pontius Navigator

23rd Mar 2016, 14:57

I recall one Nimrod case, it was in Norway when it was decided to recall it to UK. Normally it would be refuelled to full with expensive fuel but to save money it would pick up minimum route fuel. However base was closed so it would have to night stop in Prestwick, pick up enough not so expensive fuel and make the flight next day to Kinloss. So cost was X Krona of fuel, plus Y £ of food and accommodation plus Z £ of not so expensive fuel. As this was cheaper than X Krona of fuel and Y Krona of food and accommodation the crew was ordered too fly to Prestwick.

The maths took some time to calculate before they were ordered back to UK. They then played the crew duty card.

Herod

23rd Mar 2016, 15:50

you didn't need to stage at Gander when travelling west

What, and miss a night at the Flyers?

Rossian

23rd Mar 2016, 15:56

.....in 1970 during the build up to Ex Bersatu Padu ISTR. The Lightnings were being ferried out and needed a fair amount of refuelling. (Our Shacks were doing the SAR cover in mid Indian Ocean and had been gone hours before.)
There were "no go" marker posts on the runway edge to trigger an abort.
We watched as the first Victor roared off down strip at max chat, past the no go markers and appeared to fly into a huge whirling cloud of sand and appeared several minutes later and several miles out to sea desparately looking for height. There might have been a sweaty palm or two in the second one as he too used every bit of concrete and (maybe some sand) for a repeat performance.
I didn't envy them one bit.

The Ancient Mariner

Schiller

23rd Mar 2016, 16:51

RedhillPhil

Not really true, RedhillPhil. We used to launch with full internal fuel - with a catapult launch TOW was never an issue. But the SFC of the beast was appalling and we used a Scimitar to top up about 25 mins after launch to give us a decent range.

Tankertrashnav

23rd Mar 2016, 18:06

Rossian - one of our blokes took off from Dubai once and immediately realised he was struggling. No question of turning of course so all he could do is climb straight ahead with emergency 101.5 % set, hope for the best and try not to think of the JPTs. He reckoned he was 10 miles out as he passed 500'. In those days Dubai was basically surrounded by sand so luckily there was nothing to hit - I'm not sure how it would pan out these days.

Onceapilot

23rd Mar 2016, 20:29

TTN
Those parts often suffer from a strong temperature inversion that means, you can be "trapped" if power limited. This is particularly likely with military aircraft. Of course, with a known low level temp inversion, one would be wise to consider the inversion in the take-off performance calculations.:ok:

OAP

Onceapilot

23rd Mar 2016, 20:52

LW20
It is not straightforward to just lump Mil Ops into being "different" as regards fuel planning. Some Mil operations use very accurate flightplanning that is essentially identical to civvi. Most fast-jet flying is designed around the fuel load available and the task. Tankers often operate missions with 100+ton fuel loads using a totally flexible MDR plan.:) So, apart from the pre-planned Jetplan type tasks, the rest is totally flexible because........:ok:

OAP

taxydual

23rd Mar 2016, 22:15

I was told the most useless thing to a pilot.

a. Fuel in the bowser
b. Runway behind you.
c. Yesterdays met

Onceapilot

23rd Mar 2016, 22:33

I was told the most useless thing to a pilot.

a. Fuel in the bowser
b. Runway behind you.
c. Yesterdays met

Yes, along with height you've lost and, speed you've not got!:uhoh:

OAP

WIDN62

23rd Mar 2016, 22:37

Don't forget:

d. Today's met.

PARALLEL TRACK

24th Mar 2016, 13:04

e. frequency in the BINA?

Flap62

24th Mar 2016, 16:58

On the subject of fuel. If there was more than sufficient why was it said we had "gujons of petrol"? was this just a harrier thing?

Herod

24th Mar 2016, 17:49

Have experienced "a" twice and "c" twice. "I flew about learning from that"

plans123

24th Mar 2016, 18:34

WIDN62, you are absolutely spot on.I was a Mission Planner at Lyneham many moons ago and when we generated the Flight Plan, we always went out of our way to maximise the payload. More often than not this meant flag-stops in either PIK or KEF to achieve this when west bound. It was a rare occasion to bin off payload in my experience.

Onceapilot

24th Mar 2016, 19:10

is totally flexible because........:ok:

? Nobody?....

OAP

Sloppy Link

24th Mar 2016, 20:08

You can never have too much fuel.......unless you are on fire.

Courtney Mil

24th Mar 2016, 20:39

...or need to land right now!

Pontius Navigator

24th Mar 2016, 21:05

Fuel in the wings when the fuselage izs empty?

Pontius Navigator

24th Mar 2016, 21:12

TTN, do you remember the tale of the Victor, lost the feed for half the fuel?
Every so often the brains trust asked fuel remaining, divided the result, recalculated endurance.

Curiously, when the fuel remaining equalled half the original fuel unusable the engines stopped.

Rwy in Sight

24th Mar 2016, 21:26

...or need to land right now!
I thought all the aircraft are certified to land at maximum take off weight so that is not a problem.

OK465

24th Mar 2016, 21:32

Going from Tucson non-stop to San Juan, PR on a USAFR KC-135 to conduct A-7 departure training for the PRANG in their aircraft. The 135 was also dragging 4 of our F-100s to do some dissimilar after we completed the departure training.

Scheduled takeoff from Davis-[Monthan was around 0800 with the 100s going out of Tucson Int'l at the same time to join up.....

.....well, the 135 had a Mx problem (100s were held at TUS) which wasn't fixed until around 1000. when the desert temperature had gone up about 20-30 degrees. 135 guys had taken on fuel for an 0800 departure so now they had "too much", even for a 13,000 foot runway. So they took the runway and ran up the engines and we sat there shaking with brakes on for about 5 or so minutes while they 'lightened up'. When they decided they could make it they notified the Huns and off we went, actually getting airborne on the departure end overrun.:eek:

Long story short, we get about 200 miles out of San Juan, thunderstorms all around and in the SJ area, so they 'top off' the Huns, just in case. As we get closer the loadmaster comes back with wide eyes and says, 'we only have enough fuel for one approach'. We make it and taxi in and he comes back again and says, 'we need to have you all come forward so we can put the tailstand in when we park so we don't fall back on the tail'....."too little".

We had a great week there, and then a USAFR 135 shows up to take us home.

Same crew. :eek:

Fortunately we were only going to Miami on the first leg.

Courtney Mil

24th Mar 2016, 21:33

I thought all the aircraft are certified to land at maximum take off weight so that is not a problem.

Not in a full Lima fit Tornado F3 with a total utilities hydraulic failure (actually the entire contents of the hydraulic system were dumped into the airframe) with smoke and you need to get back to Decci to take a cable. Max engagement speed and mass.

Tankertrashnav

24th Mar 2016, 23:11

Curiously, when the fuel remaining equalled half the original fuel unusable the engines stopped.

So what happened then?

Actually I never heard that one, must be after my time.

Rwy in Sight

25th Mar 2016, 07:22

Courtney Mil,

Thank you Sir for learning me a new thing.

Pontius Navigator

25th Mar 2016, 08:41

TTN, no, before your time, it crashed! Crew survived. I think this is the one:
ASN Aircraft accident 14-JUN-1962 Handley Page Victor B.1A XH613 (http://aviation-safety.net/wikibase/wiki.php?id=55306)

Pontius Navigator

25th Mar 2016, 08:47

Further to CMs F3 case, it had been the norm for Vulcans to reduce AUW to below 140k. As there was no fuel jettison system the only option was to burn it off.

After the Vulcan crash at Cottesmore the lost control due to a bomb bay fire while burning off fuel Hawker Siddley said the aircraft had always been clear to !and at any takeoff weight subject to heavy landing checks.

I suspect the brains trust had forgotty this in an attempt to avoid avoidable down time for heavy landing checks. Readiness everything and cost lives and and aircraft.

Tankertrashnav

25th Mar 2016, 10:13

Thanks P-N I remeber it now. One of the very few successful rear crew abandonments from a Victor.

I love the bit about the rear crew getting the landlord to lock the pilots out of the pub!

ACW342

25th Mar 2016, 13:14

Back in the days of Ex Crusader in the mid-late 70s (Biggest UK Peacetime call out of the reserves, IIRC) there were may off-shoot exercises, one of these being Exercise Curry Express. At Brüggen 14Sqn was tasked to fly to the last man and as such their Jaguars flew with EVERYTHING hanging off stuffed into corners and as many round as the guns would take. On return Everything, including the fletchers were stripped out of the aircraft by station base engineers, as if returning from a real war sortie. If I remember correctly 14 lasted 4 days and then ceased to exist.

Those that took part that year will remember that it was a very hot German summer with temps in the 30s C (+?). In the tower it was not unusual to hear a high pitched cry of Barrier!! from a departing 14 Jag, and the departure barrier in the standby position (halfway up) would be hurriedly lowered by the local controller. IIRC the standby height of the barrier was about 8' and the aircraft crossed the barrier at about 4'

Onceapilot

25th Mar 2016, 14:34

So, apart from the pre-planned Jetplan type tasks, the rest is totally flexible because........:ok: ....because...flexibility is the key to Air Power!:)

OAP

BEagle

25th Mar 2016, 14:53

About the only thing interesting or challenging about AT flying in the VC10 was the art of fuel calculation, so that the Air Movs people could have the best possible payload offered.

Nowadays a good mission planning system can plan the minimum fuel load required for even a very complex AAR trail mission to the last kilogram in a few seconds. Either statistical, still-air or forecast met data can be used and due allowance made for contingency fuel requirements. Play tunes on prudent extra fuel requirements and payload and a good AT/AAR solution can be found, essential for a multi-role tanker transport aircraft.

A pity that some air forces still have to rely on last century planning methods though.... They must waste thousands of kilograms burning fuel to carry unnecessary extra fuel due to excessively conservative fuel planning.

I once did a transit back from Goose to the UK with a very good tailwind in a VC10K2 with a high ZFW. The very experienced navigator computed the fuel and I rounded it up a little further for a little 'wotif' comfort.. But an old Victor captain travelling in the back advised me to 'fill it up' - because that's what they always did on Victors. The Nav and I both told him he was wrong; had we followed his 'advice' we'd probably have been over max normal landing weight back at Brize....

Tinribs

25th Mar 2016, 17:25

Victor engines lived in paired nacelles so if one engine was lost it tended to throw bits into the intake of the other so in slow time shut down the engine next to the failed one
Victor built as a bomber so lots of fuel tanks in case of holes
With two engines shut down the electrics were badly short and half the fuel pumps unpowered so half the fuel became unavailable
At Gaydon 1962 engine failure shut the other one down. burn off fuel in the overhead , 60,000 lbs fuel half available 30,000 left. Some time later 45,000 lbs fuel hald available so 27,000 left. Some time later 30,000 lbs fuel half available///// quiet innit?
I joined Victors in 1966, they were still wondering how it happened

Pontius Navigator

25th Mar 2016, 20:58

Tinribs, ah, thats the one. And how iutr happened was the brains trust on the ground was asking the wrong question and cooking up with the wrong answer rather than leaving the crew too do the calculations. Then four some reason the crew based their endurance on the calculations on the ground.

riff_raff

27th Mar 2016, 02:54

"On the subject of fuel. If there was more than sufficient why was it said we had "gujons of petrol"? was this just a harrier thing?"

Didn't the Harriers have a very limited supply of water required for injecting into the Pegasus engine for cooling during a vertical landing? I have heard that the cooling water supply might only last 90 seconds when performing a vertical hover/landing at high GW.

galaxy flyer

27th Mar 2016, 03:33

OK465,

Read the Gold 11 story:

http://www.airforcemag.com/MagazineArchive/Pages/2008/August%202008/0808azores.aspx

GF

BEagle

27th Mar 2016, 06:43

I've never really understood why, according to that article, AAR was planned beyond what is described as the 'go no-go' point.

Another learning point - the plan should make sure that the receivers are airborne before launching the tanker.

But good teamwork to salvage what, to be honest, reads like a bit of a cluster.....

D-IFF_ident

27th Mar 2016, 09:14

Finding nowadays that the fuel calculation is simpler than the crew composition and crew duty requirements to accommodate a jet that could fly 24 hours or more.

Tengah Type

27th Mar 2016, 10:48

Ref #50. The Marine Corps KC130 crew were in the BX, Mess, Etc with only the Air Eng at the aircraft when drama happened. He managed to get hold of crew and had the engines running before the rest of the crew arrived to Scramble just in time to assist. The approach back to Lajes was on the fumes over the sea at VERY low altitude to the NW runway.

A potentially similar event occured to the RAF in Nov 1986. We had deployed 2 VC10K3s and 2 VC10K2s to Roosevelt Roads in Puerto Rico to recover 10 Buccs from Ex Caribbean Cutlass. The VC10K2s had to route via Lajes due to insufficient fuel to go direct to RR. The first K2 landed without a problem, but the second, which arrived 30 mins later, had to make several approaches to get in due to the bad weather that had appeared unforecast.

3 days later we launched for the recovery from RR to Lajes as waves of a VC10K2, a VC10K3 and five Buccs. Due to Wx problems there were no Jetplans or Weather info (phone lines flooded), but decision was made to launch, RV at height out of the weather and get weather forecasts en-route. There was a complete clusterf**k in the predawn darkness which caused us, in the lead tanker, to launch, only to find the rest of the wave was delayed by at least 30 mins. The formation eventually joined up 200 miles NE of RR, but still with no weather forecasts for Lajes or Santa Maria.
After the first AAR Bracket it was discovered that the Formation Fuel was very close to the minimum required to continue. Planned Tailwind was actually (unusually for the North Atlantic) a Headwind.

After the second AAR Bracket, still with no weather forecasts and still with the headwinds, the formation was now on minimum fuel to continue. Bearing mind the weather problems at Lajes outbound the only way to alleviate the situation was to reduce the formation fuel burn, which could only be achieved by one tanker leaving the formation. So after transferring all available fuel to the Buccs in an unscheduled bracket we diverted to the "tropical paradise" of Bermuda. The rest continued to Lajes where Wx was fine. The inevitable criticism of the decision which had "obviously been made in the bar the night before" was only stemmed when the Gold 11 story appeared in Air Clues a couple of months later!!

There but for the Grace of God etc.

Incidently I did get a photo of BEagle dancing with a BA hostie by the hotel pool, but since it was West of 10W it obviously did not happen! :O

Onceapilot

27th Mar 2016, 15:26

An interesting tale Tengah Type.:oh:

OAP

Tocsin

28th Mar 2016, 14:03

I'm de-lurking briefly to say thanks to LW20 for starting a thread on an aspect of military aviation that is not 'warry' - but just as interesting!

BEagle

28th Mar 2016, 14:51

Tengah Type, I remember the occasion well - for obvious reasons...

One of Lord Peter of the Bogs' more spectacularly poor trail plans!

If you recall, our other problem was that our K2 hadn't been given all the fuel we'd hoped for and also had to taxy over from the other side of the field, so we were down on fuel prior to launch as well. I say 'we', but I was only along as a spare captain cum trolley tart.

Passing the extra fuel to one of the other tankers, then diverting ourselves to Bermuda was the best plan; however, we did wonder whether the poo might hit the fan and accusations made about the reason for diverting by those who didn't know the whole picture. The sweeper Herk was so disappointed when we told him that we didn't need him, so he didn't have to divert to Bermuda....:uhoh:

Things didn't get rather more 'interesting' once the dancing was over after we didn't repair to the Bermudiana for an alternative form of 'entertainment'...:E Of course not.... West of 10°W, so it couldn't have happened. Nor did anything ever happen in Atlanta, as I'm sure you'd confirm.

Late departure the following day as it was Armistice day and the airport opened later in the morning - which begat another ching!

Happy times!!

Tengah Type

28th Mar 2016, 22:03

BEagle

As I said in my post it was West of 10W so it did not happen and RFK was not singing Irish songs. Anyway I missed it all as I was in my hotel room reading the Gideon bible. :rolleyes:
See you on the 14th perhaps?.

MSOCS

28th Mar 2016, 22:32

riff_raff, the venerable Harrier indeed did hold up to 495lbs of demineralised water. Operated or armed via an in-cockpit switch, the water was directly injected at high pressure onto the Pegasus turbine blades, cooling the JPT by around 25degC. That drop in temperature allowed more fuel to be added to reach the [now slightly higher] DECS JPT limit, whilst also increasing overall engine mass flow - i.e. increasing thrust. If used in one go it lasted 90 sec which was (normally) plenty of time to get the old girl on the deck or a pad in high OAT and/or high AUW recovery conditions, though it did concentrate the mind if you were "wet committed" and needed the water to land. Bit like the Countdown show clock running. This was routine for the Pegasus 105 engine (-406). For the more powerful 107 (-408) we hardly used the water in the UK but carried it anyway as ballast to maintain CofG margin.

riff_raff

29th Mar 2016, 03:59

MSOCS, thanks for the info. The Pegasus engine was definitely an impressive piece of design work at the time with its four lift nozzles. I believe the water was injected ahead of the 1st stage turbine nozzle to keep turbine inlet gas temps within safe limits. But as you noted, the latent cooling from the water injection allowed the fuel/air ratio in the combustor to run closer to stoichiometric.

I was involved with some of the early conceptual design work for the JSF lift fan. I wish people would appreciate the technical challenge of designing this huge 30,000 horsepower counter-rotating fan, gear drive and clutch system. The F-35B's shaft driven fan is actually much more efficient at producing vertical lift than the Harrier's use of compressor airflow.

BEagle

29th Mar 2016, 10:53

Tengah Type, I have an early start the following day, so am not sure about TBs on the 14th.

I do hope that the tranquillity of your bible-reading evening wasn't disturbed by any sounds from nearby rooms....;)

Out of interest, a trail plan I've recently been assessing required a tanker to fly across Europe, RV with 3 Typhoons on their way home, then cast them off before offering 45 min opportunity training to any 'on call' :* receivers in an AARA before landing.

Put the route in and the answer was that, without any receivers, the route required 36675 kg with 85% stat met and normal contingency fuel. Add receivers and the figure became 62382 kg with 85% stat met and normal contingency. A prudent tanker commander would elect to take more fuel to allow for wotifs, but other factors have to be considered - with average met of the day (50% stat met), the figure became 61064 kg. 'Fill it up' wouldn't be possible due to RW perf. limits; if the tanker was too heavy then the planned transit altitude might be too high and any advantage wasted.

Personally, I would have taken 66000 kg - a reasonably prudent surplus and enough for a little opportunity training. If the Typhoons had been late off and put pedal to the metal to make the RV time instead of asking for the tanker to delay at the RVIP (as a bunch of F3s once did to me....), then bin the AARA and there would still be enough to meet the trail task when contingency, planned extra and no AARA time was added together - about 8500 kg or over an hour's extra fuel for each of the receivers.

bspatz

29th Mar 2016, 18:03

Whilst managing fuel loads is obviously a prudent measure it has to also take account of the task in hand. I can recall at least one occasion when I was working in MOD when Ascot Ops contacted me to advise that the planned load couldn't go as it would mean staging Gander. Having spent some time justifying to a civil servant, who was heavily focused on costs, that it was essential that the load was moved, I politely informed Ascot Ops that the only reason that the flight was happening was to carry the load and if this meant staging Gander so be it.

Rwy in Sight

10th Apr 2016, 07:21

Alternate fields

One cool morning (while doing my military service) I overslept and I did not turn on the TACAN on the base I was stationed. The duty ATC gave me a bollocking and asked me what the military aircraft coming to land would do while the TACAN was off. I did not answer (about the alternate) and he appreciated.

So do FJ always include an alternate airport when they file a flight plan?

Pontius Navigator

10th Apr 2016, 11:12

Ri S, indeed they do. You have an emergency diversion in range of your destination and carry sufficient fuel for a procedural recovery at destination and diversion to the alternate.

Depending on distance to the alternative that spare fuel can be quite significant. In the event there is no suitable diversion there is a procedure called Island Holding but that is not valid for all aircraft and is usually only relevant for transient weather events such as local thunderstorms.

LOMCEVAK

10th Apr 2016, 11:26

PN,

Not quite correct. For fast jets, you must touch down at destination with sufficient fuel to get to the alternate, fly an approach appropriate to the weather and then touch down at the alternate with a specified minimum fuel quantity (determined by unusable fuel, gauging accuracy, c.g. limits etc and probably one visual circuit). If the weather at the diversion is colour state 'white' or better (1500 ft cloudbase, 5000 m vis) then an 'appropriate approach' will be a visual join, and if 'green' will invariably be an instrument approach. The approach requirements at destination will also be weather and approach aid dependent but if radar vectoring to an instrument approach is available then you would not plan to recover for a procedural approach.

Pontius Navigator

10th Apr 2016, 11:51

LOMCEVAK l, I was trying to keep it simple and generic. For procedure o embraced any variation on procedures and not specifically a procedural approach.

Rwy in Sight

10th Apr 2016, 21:58

LOMCEVAK and Pontius Navigator, thanks to both of you. BTW are those rules RAF or NATO ones?

Pontius Navigator

11th Apr 2016, 06:23

Both I guess but I think you will find Civair has similar rules though as you can never be sure a runway will remain open. Queue BEagle.

Where en route diversions are concerned, they may be planned and closely monitored but nit usually pre booked.

BEagle

11th Apr 2016, 07:09

On cue then, for a typical AT/AAR aeroplane:

'Final reserve' is the minimum fuel required to fly for 30 minutes at 1500ft above the alternate aerodrome.

'Alternate fuel' is the fuel required from the missed approach point at the destination aerodrome until landing at the alternate aerodrome. It takes into account the fuel required for the missed approach at the destination aerodrome, climb to enroute altitude, cruise and descent and approach at the alternate aerodrome.

'Minimum Diversion fuel' is the sum of final reserve and alternate fuel.

But then you have to take into account taxi fuel, 'trip fuel' (which is the actual fuel calculated to be used from take-off to landing at destination), contingency fuel (typically 5% of trip fuel needed from an enroute waypoint, or ideally from take-off), and for a tanker, any 'offload fuel' for receivers.

This will give you 'minimum brake release fuel', which is the sum of trip fuel, contingency fuel, offload fuel and minimum diversion fuel.

'Block fuel' is what's in the tanks prior to start. Subtract taxi fuel and the 'extra fuel' is what's left once 'minimum brake release fuel' has been subtracted.

Of course the airlines have various other rules which enable them to cut their reserves to the bone. But one major point is that the level at which an aeroplane flies from destination to alternate cannot be assured, nor can the route. So even normal legal airline planning can be caught out by ATC issues, as Ryanair found out when several of their aeroplanes had to divert on summer in Spain, following bad weather at destination. They'd all loaded a prudent extra margin of fuel, but when the air traffickers held them at much lower levels then anticipated, the crews found themselves getting rather tight on fuel. Which was emphatically NOT the fault of the airline or its crews, but was entirely due to the incompetence of Spanish ATC. The reason it hit the headlines was that they had the largest number of flights forced to divert.

Modern planning techniques should include the ability to specify the level and met conditions for the enroute phase of a diversion, as well as the desired landing fuel at the alternate (normally assumed to be the unusable fuel in tanks after final reserve has been used). This means automating the calculations provided in the Flight Crew Operating Manual, something of which traditional computer flight planning systems are not usually capable.

For example, normal planning for a 315 nm diversion assuming met conditions of the day and a climb to optimum altitude might result in a theoretical minimum diversion figure of 5945 kg. But factor in an adverse wind and a diversion held down at, say, 10000 ft and ISA +5 and this rises to 6445 kg. Which might mean arriving at the alternate with a fuel state below legal final reserve unless this had been taken into account.

For AT/AAR, accurate fuel planning is essential if the multi-role capability is to be used for a particular mission. A wet-finger guess or 'fill it up please' could well prejudice the payload available - that unnecessary extra 2 tonnes of fuel could well have been useful payload!

Pontius Navigator

11th Apr 2016, 09:11

BEagle reminds me that our Nimrod crib sheets had distance, track, height to fly and fuel required. I think there was also a headwind component factored in but it was prudent to factor in any stronger winds as usually caused diversions at Kinloss. Most diversions on Scotland were simply to TOC/TOD with no level cruise. If you were canny you might select a lower transit to benefit from a lower headwind and save on climb fuel.

We, that is some pilots, don't always follow the rules. We did a flight to Kef in the Shack. Conditions were bad at both ends and our diversion was our departure airfield. We had sufficient fuel for an instrument approach, overshoot, visual circuit and land or divert.

After the initial approach we glimpsed the airfield and the captain elected for a second instrument approach which put us below minimums. We got in. Had we had to abort I guess we might have managed Benbecular or Stornoway but he would have had some explaining to do.