Fuel Exhaustion

ZK-ANON

In this incident the aircraft ran out of fuel on base leg to its destination aerodrome.

As you will see, what the pilots thought they had in the tanks was not what they actually had.

Further Key Information

  • The only time this instructor had on type was a short type-rating and had not flown this aircraft before.
  • The instructor was operating outside of what was known/usual for them.
  • There was a poor attitude towards dipsticks prevalent in the organisation.

Investigation Analysis

This incident was investigated by the CAA and the following is excerpts from the accident report.

 

“The CAA safety investigation found that neither the instructor or the student were aware that the fuel dipstick was calibrated and marked for 'total' fuel and that 6 litres needed to be subtracted from the total fuel measurement for the C152.

 

Neither person had been taught during their training that aircraft fuel dipsticks may be calibrated in either 'useable' or 'total' fuel quantity.

 

It was also found that the fuel dipstick carried in the aircraft was for another C152 (clearly marked on the fuel stick) and after comparison with two other C152 fuel dipsticks, the one carried in the aircraft was found to be approximately 3 to 4 litres over reading at both the 10 and 20 litre graduation marks.

 

With the crew unaware of the 6 litres unusable fuel, plus the approximate 6 to 8 litre total calibration error of the fuel dipstick, the crew were unaware that the useable fuel they departed with was approximately 24 litres.

 

The crew had decided to depart with what they understood to be the required fuel requirements (if not minimum requirements) for the final leg of the cross-country flight. They had been caught out by their lack of knowledge of the aircraft fuel measurement with regards to useable fuel and the use of a fuel dipstick calibrated for another C152.”

 

Swiss Cheese Slices

What were the significant events that, if stopped, could have prevented the accident?

Time on Type

If the instructor had more time on type they would have been more familiar with the fuel system and figures.

 

Dipstick

If either pilot had been taught and understood the indications shown on a dipstick they would have not made this error.

 

Planning Rushed

If the planning was not rushed there might have been time to identify that the aircraft did not have both fuel cards as assumed.

 

Changed Plan in Kerikeri

If the pilots had stuck to the plan to refuel in Kerikeri, they would not have run out of fuel. However, it’s possible they would still be no wiser about the limitations of dipsticks.

 

Fuel Card

If both fuel cards were on the aircraft as assumed they could have refuelled at Whangarei.

 

Confirmation Bias

The gap in their knowledge about how much fuel was on board the aircraft, together with their seeking input from another instructor with exactly the same information led them, to believe they had enough fuel for the flight.

 

 

Colin Alexander, owner of Solo Wings and experienced engineer, gives some advice on dipsticks.

 

 

Causes of Exhaustion

There are four major contributing factors to running out of fuel:

  • poor planning,

  • poor inflight monitoring,

  • lack of systems knowledge, and

  • poor inflight engine management.
     

Poor planning

Time and again it comes down to poor planning, and by that we mean not thinking of all the things that could go wrong and having contingencies in place, and not having the right information in order to plan comprehensively.

 

Fuel Plan

 

Every fuel plan must include a fuel allowance for the following:
 

  • Contingency – often 10% to allow for the book figures being incorrect.

  • Ground delays/taxiing – especially if you are operating from a controlled aerodrome.

  • Fuel for an alternate – not usually required for a VFR flight, but worthwhile if you think the weather is less than ideal.

  • Weather diversions – better to have some fuel on board for this, than not.

  • ATC delays – if you’re planning to enter controlled airspace, you may need to hold before you are allowed in.

  • Holding – this can be quite normal outside a control zone, if it happens regularly, add some fuel.

  • Increased headwinds – when was the last time your groundspeed was exactly as you planned? Wouldn’t it be better to expect the best but plan for the worst?

  • Variable mixture management – if you do not lean the mixture during cruise your engine will use more fuel than the performance figures suggest.

  • Accurate fuel burn figures – know what it actually burns and take the time to cross-check and establish those figures.
     

Reserves

These are only to be used when you encounter a genuine surprise. They are not for holding, not for weather diversions, not for increased headwinds, and not for ATC delays. These should all be planned for in your fuel plan. Reserves are for the time the grass mower breaks down in the middle of the strip, or an earthquake happens and the runway needs to be inspected, or another aircraft has an emergency and you are delayed.

 

The legal day VFR minimum reserves for aeroplanes is 30 minutes. This does not mean it must be your minimum. Do you really want to land with only 15 minutes in each tank? In a Tomahawk that’s 6 litres a side. Do you trust your dipsticks to measure that accurately?

 

Any time you think you are going to need your reserves, you should be very nervous and actively putting together a Plan B that gets you on the ground as soon as possible.

 

Fuel on Board

If you don’t know how much fuel you have on board, your planning is almost worthless. You must know how much is in the tanks, exactly, before every flight. And even better, is knowing how many minutes is in each tank. Use the Time in Your Tanks card (or other similar method) to monitor and track your fuel usage during flight.

 

Dip the tanks just prior to flight, not only after you fuelled. There are too many instances of pilots getting airborne without realising fuel has been venting out of the tanks while they had their lunch. Or, alternatively had fuel stolen overnight.

 

Dipsticks

Using a fuel dipstick is the most accurate way of determining the fuel on board, although some electronic systems are very accurate so long as the correct information has been entered.

 

As you have seen above it is important to ensure you have the correct dipstick for the aircraft. Keep the dipstick clean (don’t place it on the ground, for instance). It is often said that there are only two places a dipstick should ever be – in your hand or in the aircraft seat pocket.

 

Most dipsticks in New Zealand have been specifically calibrated to the fuel tanks of a particular aircraft and are not interchangeable with those of any other aircraft, even of the same type. Check the registration marks on the dipstick every time you use it.

 

Unfortunately, dipsticks are sometimes in poor condition and their markings hard to read. If that’s the case, double-check readings to make sure that they seem sensible and arrange to get the dipstick re-marked.

Know whether the dipstick shows usable or unusable fuel – or both.

 

Dipping the Tanks


The aircraft should be parked on level ground. If that’s not possible, dip each tank, turn the aircraft through 180 degrees, dip each tank again, and take the average of the two values. It may not be accurate, but it will be better than either of the two single readings.

 

Make sure the fuel system is not cross-feeding during fuelling. Slope and uneven fuel quantities in each tank can cause this on some aircraft. With the fuel selector set to BOTH, the tank you’re filling can be cross-feeding to the other. By the time you’ve finished filling the second tank, the amount of fuel in the first tank will have reduced. It should be checked again and topped up as required.

 

The dipstick should be inserted in the filler neck perpendicular to the wing surface, unless another method is specified in the Flight Manual. Quickly withdraw the dipstick and check the indicated fuel level before evaporation, or ‘wicking’ occurs.

 

Fuel tanks should always be dipped after refuelling to establish the exact amount of fuel on board, even after adding a known quantity of fuel, and then again just before flight if you have left the aircraft for a length of time.

 

Do not rely on someone else to confirm the state of your aircraft’s fuel. You are the pilot-in-command, it is your responsibility and we have seen a number of cases where what was said by someone else was not what was heard by the pilot-in-command.

No Dipstick?

If the fuel tanks cannot be accurately or easily dipped, start the flight with the tanks either full, or filled to a fixed reference point, and keep an accurate in-flight fuel log. If that’s not possible, due to weight and balance or performance considerations, the only way to know exactly how much fuel is on board is to add a known quantity (ie, a reading taken from the fuel pump counter) to a predetermined reference point inside the tank. Consult the aircraft Flight Manual for specific details.

 

Unusable/Usable Fuel

Understanding the difference between the terms ‘usable’ and ‘unusable’ fuel is critical for determining the fuel available for flight.

 

The amount of unusable fuel can vary considerably from aircraft to aircraft, the Flight Manual should include specific figures for this.

 

The usable fuel is the quantity of fuel available for flight planning purposes. That’s the only figure that should be used when calculating fuel endurance. Many dipsticks are calibrated to read the total fuel quantity in the tank, which means the unusable fuel must be subtracted to determine the fuel available for flight.

 

As the accident above shows, when you mix up the two it can mean the difference between landing safely and landing one paddock short!

 

Fuel Gauges

 

Most fuel gauges can read reasonably accurately, but if they don’t, they must be fixed. Gauge accuracy can easily be checked before the flight by dipping the tanks (if that’s possible) and comparing the figures with the actual gauge readings.

 

The design of some tanks means you can’t get an accurate dipstick reading at certain fuel levels, so the accuracy of the fuel gauges becomes even more important.

 

You should also monitor the gauge during flight, and compare it with your flight path. Does it make sense that you’ve used this number of litres of fuel for the distance you’ve flown?

 

Poor Inflight Monitoring/Fuel Appraisal

Knowing how many minutes is in each tank, at all times, will mean every time your plan changes you will know how that affects your fuel.

 

Use a fuel log to keep track of which tank you are using and the number of minutes left in each tank.

 

Fuel Log

Whichever method you use, your log should contain the following elements:
 

  • Fuel required
    This is the fuel necessary for warm-up and taxi, climb, each leg of the flight, legal reserve, and a contingency (variable reserve).

  • Fuel available
    This is the total usable fuel carried and the endurance – the usable fuel less the extra fuel required for warm-up, taxi and climb divided by the cruise consumption rate.

  • Endurance
    It’s also good practice to record the safe endurance, which is the fuel endurance less the legal reserve. By adding this safe endurance to the takeoff time, you can record a ‘land-by’ time once the flight begins – that way you will not be tempted to try to get to the destination using the legal reserve.

  • Tank selections
    There should also be a section in which to record the time each tank selection is made, and a running total of the fuel remaining in that tank.
     

It’s also critical to factor fuel into any changes you make in your flight. Whenever your ETA changes – think fuel.

 

Here’s an example of a fuel log with the calculations for a ‘land-by’ time.

 

 

Having a fuel log is important for every flight, but just as important is to take the opportunity to double check your figures. When you land dip the tanks and check that what you have in the tanks is what you expect to have from your log and what you read off the fuel gauges. If not – search out the answer.

 

Lack of Systems Knowledge

Too many accidents can be traced back to not knowing the fuel system well enough, and this can be traced back to less than thorough type-ratings.

 

From dipsticks to drains – know how it all works. Closely study the Flight Manual or pilot notes.

 

Of particular interest to you should be:
 

  • Fuel grade, total capacity, usable, and unusable fuel quantities.

  • Fuel drain points and fuel tank dipping procedures.

  • Fuel selector operation, especially any cross-feeding procedures.

  • Electric and mechanical fuel pump operation, and normal fuel pressure and fuel flow gauge readings.

  • The purpose of fuel boost pumps.

  • Correct leaning procedures, and consumption rates for different altitude and rpm combinations.

  • Manifold pressure and rpm for maximum range or endurance.

  • Learning the engine trouble checks.

 

It’s also really easy to assume one aircraft is the same as another.

 

To help you truly understand the fuel system of the aircraft you fly we have developed an app Know Your Aircraft which tests your knowledge and lets you build a comprehensive schematic of the fuel system for each aircraft you fly. Work through it in consultation with the Flight Manual, an instructor, and maybe an engineer. You can download this app from the Google Play Store and the App Store for free.

Poor Engine Management

The performance figures in the Flight Manual are often predicated on the leaning of the engine. Remember to lean any time you are cruising at less than 75% power – at any altitude, not just above 3000 feet.

 

On some aircraft, a properly leaned engine can increase your still-air range by as much as 20% compared with not leaning at all at the same altitude.

 

A combination of increased rpm and incorrect leaning, however, could increase consumption by as much as 15%. That equates to a 45-minute reduction in endurance – there goes your reserve, and then some.

 

Leaning procedures vary considerably between aircraft. Some engines have very basic instrumentation and require the pilot to lean the mixture by ear and reference to rpm, others have exhaust gas temperature and fuel flow gauges, which allow a far greater degree of accuracy.

You must be familiar with the correct leaning procedure for the aircraft you fly.

 

It’s also important to note that leaving the carb heat ON enriches the mixture and consequently increases the fuel burn.

 

Colin Alexander, owner of Solo Wings and experienced engineer, gives some advice on leaning and priming.

 

Low Fuel State

If you do end up with lower than comfortable fuel – declare an emergency – do not suffer in silence. It is better to be a little embarrassed than stoically dead.

Then preserve as much fuel as you can:

 

  • lean the engine – any time you are cruising at less than 75% power

  • fly at the best economy speed – do you know what that speed is? Could you set it automatically and then check it in the Flight Manual?

  • Stay as high possible for as long as possible – the higher you are the less the fuel burn, and the further you can glide if the engine does go quiet.
     

When you are low on fuel, there can be warnings in the Flight Manual about not carrying out uncoordinated flight manoeuvres like sideslipping as it can unport the fuel lines – causing the fuel to slosh away from the fuel lines and starve the engine of fuel. Adding just that little bit more stress when already low on fuel.