You’re pointed away from the destination airport on some controller’s vector and you are sweating the near-empty fuel gauges. You can’t be certain when you’re going to be turned toward the airport and how long it will take to get there. Show As a last resort you tell the controller you are minimum fuel and need priority to the runway. Did you violate FAR 91.167, the rule that sets the requirements for minimum fuel when flying under IFR?  Does this violate the FARs? The only certain answer is “maybe.” The reason is that the FAR describes required flight planning, not the actual amount of fuel in the tanks. If you made a credible flight plan and fueled the airplane accordingly, but some unforeseen event caused you to run short of fuel, you may not have violated the rules. Even more confusing is the title of FAR 91.167, which is “Fuel requirements for flight in IFR conditions.” Look at FAR 1.1 definitions and it says “IFR conditions” are weather conditions below VFR minimums. If you’re flying on an IFR clearance in VFR conditions do the minimum IFR fuel requirements apply? Or is it the lesser minimums for VFR fuel under rule 91.151? I don’t know. And that’s why if you make it to a runway without losing power from fuel exhaustion you may not have violated any FAR. Another potentially confusing component of the minimum fuel requirements in the FARs is the definition of the quantity of fuel required. For both VFR and IFR flight planning the reserve fuel is expressed enough fuel to fly for either 30 or 45 minutes at “normal cruising speed.” What the heck is “normal cruising speed?” Most airplane flight manuals or pilot operating handbooks show true airspeed and fuel flow per hour for three cruising conditions—normal, long range, and high speed. Sounds simple enough. Look at the chart labeled normal cruise, multiply the hourly fuel flow by .75, and that’s the FAR required reserve fuel amount. But that chart will show cruise and fuel burn at a range of altitudes across the airplane’s operating envelope. And the airspeeds and fuel burns will be dramatically different at various altitudes. That’s particularly true for turbine-powered airplanes where fuel flow can easily be double, triple, and more when flying at 3,000 feet instead of a typical cruise altitude far up in the flight levels. If you want to fly by the rules you could legally plan a flight with a reserve that represents just 45 minutes of fuel burn at an optimum normal cruise altitude as reserve. Legal, yes, but that makes no sense. The intent of reserve fuel is to accommodate the unexpected. A wrong forecast for winds aloft en route can make hash of any flight plan. So can an unexpected and longer routing clearance from ATC. Unfavorable altitude assignments, particularly in turbine flying, can also burn up extra fuel at a furious pace. Then there is always the possibility of air traffic and airport system failures. If an airplane slides to the edge of the runway at your destination and wipes out the gear it’s going to take a long time to clear it. This time of the year over much of the country snow removal activity can close an airport for an extended and impossible to predict time. And there is always the possibility of equipment outages in the ATC system that create considerable delay. That happened to me not long ago on a trip into Bozeman, Montana. While we were still about a half hour out, Big Sky Approach Control lost its radar. That meant airplanes had to be separated “manually” in the terminal area. So we sat in a hold at an initial approach fix for nearly 40 minutes, waiting our turn for the approach. The weather was well above minimums, but until the tower had the arriving airplane in sight, the next airplane couldn’t be cleared for the approach. That would have been a real crisis if only the legal 45 minutes of fuel had remained at the destination. The best news is that the computerized flight planning services—supported by really terrific improvement in winds aloft forecast accuracy over the last 20 or 30 years—has made precise fuel planning a snap. What is “normal cruising speed” down low? My favorite flight planner is fltplan.com. Many swear by ForeFlight. Over the years flying a variety of airplanes I’ve found fltplan.com to be uncanny in its precision. It is a big surprise if fltplan misses time en route by five minutes on a three hour trip. And fuel burn predictions are reliably within a few pounds. But fltplan plans by the rules. It calculates fuel burn based on route, cruise altitude and winds forecast. It does the same to plan fuel requirement to fly to the destination, and then the fuel needed to fly to the filed alternate airport. And then it adds in the amount of taxi fuel you have specified and calculates 45 minutes of fuel burn at your selected cruise altitude and power as the FAR-required reserve fuel amount at the destination or alternate airport. Fltplan shows this fuel total as “minimum dispatch fuel,” and the emphasis is on minimum. It’s the least legal fuel you can depart with based on the considerations of winds and routing. And that’s the starting point. For example, in the King Air 350i that I fly, fltplan calculated a reserve of 512 pounds for one trip, and 441 pounds for another leg on the same day. The difference was created by planning a higher cruise altitude on the second leg, where fuel flow would be lower at normal cruise. So fltplan did the “legal” calculation and included 45 minutes of fuel at the higher altitude where I planned “normal cruise.” The real fuel load I want includes a fixed reserve, not the “legal” amount. My real reserve is one hour of fuel at a realistic cruise fuel burn. In the King Air 350 that’s 750 pounds of fuel. And even that number is not truly fixed in my planning because doubts about weather forecasts, or bizarre routing that you can get in busy airspace, or unfavorable altitudes along many routes, add to my reserve. It’s tempting to think the FARs found in Part 91 are conservative, even safe, but they are really bare minimums in many instances. And fuel reserve planning is one of those. When people ask Mac McClellan what he does for a living, he replies, “I fly airplanes and write about them. And I’m one of the most fortunate people in the world to have been able to make a career of doing what I love.” Mac has been a pilot for more than 45 years, an aviation writer for more than 40 and has been lucky enough to get to fly just about every type of personal and business airplane in production from the 1970s onward. He was on the Flying Magazine staff for 35 years and editor-in-chief for 20 of those years. He has private pilot privileges in single-engine airplanes, commercial pilot in helicopters and ATP in airplanes with more than one engine. He holds several business jet type ratings and has logged more than 10,000 hours. His first airplane was a Cessna 140 and for the past 27 years he has owned a Baron 58 flying it more than 5,000 hours to cover the aviation industry. And now he is a part-time corporate pilot flying a King Air 350. The fuel carried by aircraft is highly regulated. It is commonly believed that an aircraft carries only the fuel that is required to fly from one airfield to another airfield. But this is not entirely true. Aircraft, under commercial operations, carry a lot more than that. The fuel policyThe amount of fuel to be carried for a flight depends on local air regulations. The fuel policy must be written by the airline in their operations manual based on these regulations. Generally, the policy must be based on:
The fuel for a commercial flight is divided into taxi fuel, trip fuel, contingency fuel, alternate fuel, final reserve fuel, additional fuel, and extra fuel.
The use of SAF is a growing trend in the airline industry. Photo: Getty Images. Taxi fuelTaxi fuel is the amount of fuel required to taxi the aircraft from the gate to the runway for takeoff. It also includes fuel burnt at the gate due to the operation of the Auxiliary Power Unit (APU).
Trip fuelThis is the fuel required to fly from the destination aerodrome to the arrival aerodrome. It should include:
Trip fuel is the fuel required to get from origin to destination airport. Photo: Vincenzo Pace | Simple Flying Contingency fuelAs the name suggests, contingency fuel is the fuel that is uplifted to account for unforeseen circumstances such as changes in the weather, route changes due to ATC constraints, holding, etc. The contingency fuel should be higher of:
Contingency fuel accounts for unexpected weather. Photo: Getty Images. Alternate fuelThe alternate fuel is the fuel required to divert at the destination if the pilots fail to land at the destination for some reason. The alternate is chosen by the airline before the flight, and sometimes two or more possible alternates are provided in the flight plan. The fuel for alternate should include:
Airbus<\>" data-modal-id=single-image-modal data-modal-container-id=single-image-modal-container data-img-caption=""> Final reserve fuelThe final reserve fuel is the absolute minimum fuel that is required for an aircraft to remain airborne safely. Ideally, pilots should not ever reach this fuel. If, for some reason, the pilots feel that their fuel load will drop to final reserve, a Mayday should be declared. The final reserve for an aircraft with turbine engines is the fuel that is required to fly for 30 minutes at holding speed at 1500 ft above the alternate aerodrome. The aircraft weight to calculate this fuel should be not less than the weight that is estimated that it will be when it reaches the alternate. To be conservative, airlines can choose a higher weight as increments in weight increase fuel burn. For an aircraft with piston engines, the fuel is that required to fly for 45 minutes. This difference is because turbine engines are more reliable than piston engines. Additional fuelThe additional fuel is an interesting one. Not all airlines, or to be more accurate, not all routes, have additional fuel as a minimum fuel requirement. However, certain operations, such as ETOPS, require airlines to carry additional fuel when flying ETOPS routes. This is the fuel required to divert to an ETOPS alternate with engine failure and/or loss of pressurization. This is the extra fuel carried at the discretion of the captain of the flight. The captain is authorized to carry more than the flight plan fuel based on his judgment. However, many times airlines require captains to give justifications for any extra fuel that is carried.
The captain decides the extra fuel required. Photo: airBaltic If you remember, we have previously said that a minimum of 5% of trip fuel must be carried as contingency fuel. It also stated that if the inflight replanning procedure is followed, you only need 5% required for the remaining part of the flight. The 5% may not be an issue for a short flight. However, for longer flights with higher trip fuels, the contingency fuel can skyrocket. So, how does RCF or inflight replanning work? Let us say, you are planning to fly from destination A to B. And when the fuel is calculated, it shows that 80,000 kg of fuel is required to fly from A to B (trip fuel). Now, as a contingency of 5% of this trip fuel is a part of the minimum fuel to be legal you must carry an additional 4,000 kg of fuel (5% of 80,000 kg). As you can imagine, this is a lot of fuel. Now, if you want to reduce this contingency amount, you could use the RCF or the inflight replanning procedure. To follow this procedure, you can find an aerodrome that is closer to A than B. We will call this aerodrome C. You then find out that to fly from A to C, only 50,000 kg of fuel is needed. This means your contingency now falls to 2,500 kg. So, you will initially make a flight plan from A to C to fly to aerodrome B (your original destination). This flight plan must include a decision point, maybe close to your TOD to aerodrome C. At this decision point, you make a quick fuel calculation to see whether you have enough fuel to reach aerodrome B from the decision point plus 5% of the fuel that is required to fly from the decision point to aerodrome B. If it is determined that you have enough fuel as above, you will simply do an inflight replanning to destination B. The leeway given by the regulators is that if such a replanning inflight is done, at the decision point, you are only required to have 5% of the fuel that is needed to fly from the decision point to the destination (in our case aerodrome B).
Photo: Anas Maaz - Simple Flying So, how is the minimum fuel for such a procedure determined? It should be the greatest of a) or b) a)
b)
Inflight replaning saves airlines a lot of money. Photo: Vincenzo Pace | Simple Flying Predetermined Point (PDP) ProcedureWhen flying to places that are super isolated, an alternate aerodrome can be quite far from the destination. In these situations, the flight can only be routed via a predetermined point. The predetermined is the point where the fuel in the aircraft is only enough to reach either the destination aerodrome or the alternate aerodrome. So, before the PDP, pilots should make a calculation to make sure they have enough fuel to continue to the destination.
Photo: Anas Maaz - Simple Flying To apply this procedure, the fuel should be the greatest of a) or b) a)
b)
The airline dispatch office prepares the flight plan, which specifies the minimum fuel required to fly from one aerodrome to the other. This fuel is based on current wind and temperature data. So, it is fairly accurate. However, pilots, many times, take extra fuel for their comfort, particularly when the weather is bad.
The pilots decide the fuel based on dispatch data. Photo: Airbus To calculate how much they can carry, an underload can be calculated. The underload is the amount of weight that can be carried in the aircraft without exceeding any limiting weights with passengers and cargo loaded. For an airliner, there are three main limiting weights. The Maximum Takeoff Weight (MTOW), the Maximum Zero Fuel Weight (MZFW), and the Maximum Landing Weight (MLW). It is always easier to work out an example. Say, an aircraft has weights as follows:
Current Zero Fuel Weight (ZFW): 80,000 kg The fuel is as follows: Block fuel (total fuel) = 15,000 kg Trip fuel = 10,000 kg Taxi fuel = 300 kg The underload calculation:
According to the above calculations, the limiting weight is MTOW, and the underload is 5,300 kg. What this means is that if more than 5,300 kg of fuel is uplifted, the MTOW will get exceeded. So, in this case, the pilots can carry 5,300 kg of fuel without busting any weight limitation of the aircraft. |
What is the minimum fuel reserve for day VF our flight and on what cruise speed is the fuel reserve based?
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