Fuel Starvation

ZK-ORN

This accident caused a media stir when it was plastered all over the international news and internet, but what’s really important is how the aircraft ended up on the beach in the first place.

Further Key Information

Civil Aviation Rules allow microlight owners to do their own maintenance, and this pilot and his friend had been maintaining and flying this aircraft together for many years.

 

Investigation Analysis

The beach takeoff accident was looked into by the CAA and it was relatively easy to see why the accident occurred, but what proved more interesting was how the aircraft ended up on the beach in the first place.

 

Swiss Cheese Slices

What were the significant events or actions that, if carried out, would have prevented this incident.

Functional Check

At no point after the maintenance was a complete system check done to ensure the systems that had been altered or replaced were operating correctly.

 

Having a system and worksheet for the maintenance they were carrying out may have helped.

 

Tank Contents

Because fuel had leaked out of the left-hand tank while they were replacing the fuel drain the pilots did not notice that the fuel level in that tank had not reduced during the previous test flying.

 

Assumed System Worked

The engine ran when started and the test flights were completed without incident. This led the pilots to assume everything worked as they expected.

 

Checking the fuel flow from each tank would have identified the lack of fuel coming from the left-hand tank to the engine.

 

Distracted and Delayed

Replacing the fuel drain on the day of the flight led the pilots into being distracted with the mess it made and delayed because it took them longer than anticipated. This meant they did not pay as much attention as usual to their preflight preparations.

 

Inflight Fuel Monitoring

During our interview the owner admits he glanced at the sight gauges and thought that the right hand one was not looking like he expected, but because it didn’t fit with his mental model, discounted the reading. However, moments later the engine stopped, forcing him to focus on carrying out the forced landing onto the beach.

 

If this monitoring had been carried out a little earlier, it would have become apparent that the fuel levels were not as expected and that there was a problem.

 

Causes of Starvation

The causes of fuel starvation are very like those of exhaustion, with perhaps more emphasis on inflight system management, since the definition of fuel starvation is that there is fuel onboard the aircraft, just not getting to the engine.

 

Starvation tends to be the result of:

  • a tank being inadvertently run dry, because of distraction or an incorrect fuel selector position – something you can prevent,

  • a venting or cross-flow problem – something you may have been able to prevent, or

  • a mechanical problem such as a stuck fuel valve – something you probably can’t.
     

Poor Planning

 

Again it comes down to poor planning, by not understanding what reserves are for, running the aircraft too low on fuel, or knowing exactly how much fuel you started the flight with.

 

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 enough?

 

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.

 

Unporting

By flying out of balance the fuel can move to the outer part of the tank and leave the outlets sucking air and not fuel - unporting. So when the fuel levels in the tank are low it can be deceptively easy to unport the tank outlets. Some Flight Manuals specifically prohibit uncoordinated flight when fuel contents are low.

 

If you only have 6 litres in a tank, it doesn’t take much for it to slosh away from the outlet.

 

Fuel on Board

You must know how much is in each tank, 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 a tank while they had their lunch.

 

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. We have seen a number of cases where what was said by someone else was not what was heard by the pilot-in-command.

 

Poor System Management

Knowing the aircraft fuel system is vital to managing your fuel usage to ensure you don't run out.

 

Pre-Takeoff Checks

The worst possible time to have an engine failure is just after takeoff. Quite often this is caused by a fuel problem.

 

Commonly this happens when either a nearly empty tank is selected for takeoff, the fuel selector gets stuck between tank settings, or is selected OFF, which provides just enough fuel to taxi and get airborne.

 

Those types of incidents can be avoided if the pre-flight and pre-takeoff checks are carefully followed. Here are some suggestions for those aircraft with fuel selectors and boost pumps.

 

  • Always check the fuel selector is securely in the detent for the required tank.

  • Select the emptiest tank for engine start. This will ensure there are no fuel flow problems with that tank.

  • Before starting the engine confirm the electric fuel pump is operating by listening for a ticking or whirring noise, and by looking for a slight rise in fuel pressure.

  • After starting, confirm the engine-driven fuel pump(s) are operating, by making sure the fuel pressure is stable when the electric fuel pump(s) are turned OFF.

  • Change to the fullest tank before the engine run-up. This allows time for the fuel flow to stabilise and for any contaminants to pass through to the engine before the takeoff. Now do both a visual and a hands-on check of the fuel selector(s) to minimise the chances of a mis-selection.

  • Double check that the fuel gauge readings for each tank are what you think they should be, that the electric fuel pumps are on, and that the fuel pressure is normal.

  • Quickly scan the fuel pressure at the start of the takeoff roll. Wait until you have cleaned the aircraft up, with climb power set, and are at a safe height before turning off the fuel pump(s).
     

Tank Selection

 

As we have said before, be very careful about which tank you select. And when you select it.

 

Don’t change tanks at a low altitude, like just before or after takeoff, just because the checklist says to change tanks. It can be very easy to fall into the complacency trap and simply go through the motions without thinking critically about which tank you want and why.

 

This is also true for the before landing checks. Quite often they will say ‘Fuel Tank – Fullest’ and it becomes a habit to simply change tanks at this point. Don’t let blind habit catch you out.

 

Venting and Cross-feeding

The integrity of fuel tank vents should be checked during the pre-flight inspection. A blocked or deformed vent – and there have been cases of insects building nests inside fuel vents – will mean the engine-driven fuel pump has to work very hard because the fuel tanks are unable to equalise with the atmosphere. That could eventually result in a collapsed fuel tank and possibly engine failure.

 

Colin Alexander, owner of Solo Wings and experienced engineer, gives some advice on the dangers of venting and cross-feeding.

 

 

In a recent example of cross-feeding, an aircraft was refuelled on arrival at Queenstown. The pilots left the aircraft parked while they had lunch. They then departed for Omaka. Enroute they encountered weather delays and were approaching ECT so opted to land at Woodbourne. Unfortunately, they ran out of fuel on short final and managed to put the aircraft down on the only paddock without grapevines just short of the threshold.

 

On investigation, it seems fuel had been leaking out of the aircraft while it was parked. The pilots had not dipped the tanks before departure so where unaware of the reduction in available fuel. These pilots were very lucky they could land in the only paddock for miles without grapevines.

 

Unusable Fuel

The amount of unusable fuel can vary considerably from aircraft to aircraft – refer to your aircraft Flight Manual which should show specific figures. The fuel tank outlets on some aircraft types are very susceptible to becoming un-ported during prolonged unbalanced flight, which eventually leads to fuel starvation and engine failure.

 

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

By keeping an accurate fuel log you will have quick access to information on your fuel state. Here is the example we showed you in the Fuel Exhaustion section.

 

In particular, we are interested in the red square – the inflight monitoring section. Keep this updated and make sure to note the time of your tank changes. The numbers in here should be in minutes. This example shows 75 minutes in each tank.

 

One particularly common method for keeping your fuel tanks balanced (when taking off with the same amount of fuel in each side) is to wait half an hour before changing tanks and then change every hour after that. This will keep your tanks within half an hour of each other and more-or-less balanced.

 

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 that 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. This app can be downloaded from the Google Play Store or the App Store for free.