Final reserve fuel based on archaic data: Unsafe for future growth mindFly

There can be a finite amount of fuel in the tanks of an aircraft and the minimum required to commence a flight is defined in the International Civil Aviation Organisation (ICAO) Annex 6 document and DGCA regulations.  Over the years we have witnessed increased incidents of low fuel scenarios.  These could be a result of a combination of a number of factors.  The actual minimum fuel requirements defined by the ICAO/DGCA may be sufficient if the present accident rate is accepted. Europe has a stringent requirement of keeping the accident rate below 20%, which translates to less than one accident per ten million commercial flights, keeping the growth rate until year 2050. Studies in Europe show the current fuel policy is not sufficient to guarantee the long-term safety target for Europe. The very competitive situation in the airline market incentivizes the use of creative interpretation and circumnavigation of existing requirements in order to achieve small cost reductions by decreasing the fuel uplift. Therefore, it is reasonable to assume that airlines under high cost pressure will use strategies to reduce the arrival fuel as much as possible, if legally permissible. For example, in Europe regularly using the alternate fuel to cover expected delays during the flight is not desirable, but is also not illegal under current legislation.

Today, fuel emergencies already occur and the probability is high, particularly when air traffic is affected by external disturbances, such as thunderstorms or unexpected traffic congestion. 

A flight dispatched with the flight plan fuel and 30 min of final reserve fuel should have the confidence of the flight crew with respect to the accuracy of data and predictions. Humans are conformists and would most likely believe what is given in the tables generated by a software. A delay in departure or enroute will affect the final reserve but most flight crew would continue the flight rather than refuel for a few hundred kilos.

Current regulations stating that the Final Reserve Fuel which is the fuel required to hold over the alternate after a diversion from the destination, must allow for 30 min of flight time cannot guarantee that an averagely performing flight crew will successfully handle complex failure scenarios. Given the relatively low complexity of first and second generation jet aircraft compared to today’s aircraft, ensuring that additional time is available to handle a technical problem was considered less important. At that point in time (50 years ago), the complexity levels of abnormal or emergency procedures were relatively small. Also, the total number of flights only amounted to 5% of today’s traffic (ICAO, 2016 data). Therefore, severe traffic congestion at airports was nonexistent. The question now arises as to whether the requirement on Final Reserve Fuel is still up to date, or whether the complexity and length of abnormal procedures, the traffic volume and today’s safety targets requires a modification. The Safety Management Manual (SMM) published by the ICAO requires airlines to identify hazards and unsafe conditions, so-called emerging risks, that have not yet caused an incident or accident (ICAO, Annex19, SMM). Emerging risks should be reviewed and, if necessary, corrective actions have to be defined to take control of these emerging risks (ICAO SMM). This is achieved by performing quantitative risk assessments, including studies and experiments. The risk assessments must demonstrate that a proposed change in the aviation system does not increase the probability of an accident.

Studies show that the average time to handle a complex failure in a modern jet aircraft is 26 minutes. This, along with the complexities in the air traffic congestion and other peculiarities of the airport and approach aids can lead to a fuel starvation. 

Fuel starvation scenario

With a remaining period of 30 min, as specified by the current fuel regulations, 18% of the crews (see right y-axis) within an error interval of [10%; 40%] would not have had enough time to handle the problem. An error interval of 3 min is considered, indicated by the red and green error bars, to account for uncertainties: firstly, there may be a discrepancy between the indicated fuel and the usable fuel, which are roughly 3 min flight time (Langton et al., 2009). In the past, engine flameouts have been observed before the fuel quantity indication showed zero (CIAIAC, 2010). Secondly, regarding traffic at major airports, the route to the airport or speeds assigned by ATC may not correspond to the planned optimum, which is the basis for fuel calculation. By contrast, with 45 min of available flight time, the probability of fuel starvation is reduced to approximately 2%, a reduction of around 90%, assuming that complex Failure does not change.

Cost impact of the higher fuel requirement has also been analysed.  The extension of the final fuel reserve from 30 min to 45 min is a highly cost effective measure for mixed operations, and would even be more effective when considering short-haul flights alone.

From the ecological aspect of carbon footprint, in the long run, the additional emissions are compensated by the latest aircraft generation like the B787 and A320 NEO that promise up to 20% less fuel burn.

The study concludes that an increase in the final reserve fuel from the current 30 minutes of hold fuel at the alternate to 45 minutes is highly desirable to ensure the acceptable level of safety.   This caters for the growth in air traffic, increased complexity of the failures aboard aircrafts as compared to the 50 year old data used for drafting the current requirement, cost factor and ecological impact.  

7 Comments

    1. The worst case on a A320 would be diversion to alternate and having to deal with a dual hydraulic failure. There is an enormous amount of ECAM actions, paper checklist to go back and forth. it takes time on an airbus.
      Training scenarios focus on failure after takeoff and not close to destination or alternate where fuel is critical too.

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    1. Agree, UK is looking at fuel capability alternates. Unforeseen situations cannot be catered for at all times. The article focusses on planned alternate with a possibility of a technical failure. Getting out of the congested airspace following a diversion also takes up some time and thus fuel.

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  1. In today’s environment it is very important that while accepting ATS flight plan by ANSP, airfield capacity to handle is not only checked for destination but also for destination alternate. This will ensure that destination alternate is not overwhelmed by multiple aircraft declaring May Day due breach of final reserve fuel, in case of multiple diversions.

    Liked by 1 person

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