It might seem like an answer only pilots need to know, but based on my own personal experience having worked as both a Flight Dispatcher and an Aircraft Fueler, they too should be aware of the APU burn rate. So how much fuel does a B737 APU burn?
First of all, why would a Flight Dispatcher need to know the APU burn rate? The flight planning software automatically adds fuel for taxiing on top of the required fuel for the planned flight. So whats the big deal?
In my experience, during winter deicing operations, aircraft sometimes must wait a while to be deiced after being pushed back from the gate. (This does not apply if deicing is done at the gate before push back.) Typically aircraft ranging from regional jets to narrow bodies can be fully deiced within 10 to 15 minutes. Depending on how much snow and ice has accumulated on the aircraft and how difficult it is to remove. Also if Type 4 needs to be applied, you should expect an even longer wait. With airline schedules, typically multiple flights require deicing within a short period of time from one another. As a result, the deicing queue can get long very quickly.
As you know, once an aircraft has pushed back, the APU needs to run to power the aircraft and later start the engines after the deicing is finished. (Assuming the APU is operative. If the APU is inop, then at least one engine will need to run instead.) As long as the APU or even an engine is running, that aircraft is burning through fuel.
When a flight plan is generated, the required fuel for the flight is calculated based on the current conditions to meet the legal fuel requirements. Flight planning takes into account things such as Weather, NOTAMS, MELs, Company Policies, Payload, etc. In the end, a flight plan states a specific amount of fuel that is required to be onboard the aircraft when it becomes airborne. I knew this as “minimum fuel” or Min Fuel. If you have 1 lbs or 1 kg less than the legally required fuel, your legally required to return to the gate to get more fuel. Obviously this is a situation you want to avoid.
If your thinking, that’s no big deal, just quickly return to the gate, grab more fuel and head back out again, she’ll be right. Um no, that’s not going to work very well and here is why.
What type of deice fluid did you get? Is it still precipitating? When does your Hold Over Time expire? Is the gate you pushed from still available? Are any other gates available for you to return to? Is the ground crew who worked your flight still available? (Or any ground crew) They may have likely moved on to another flight also running on schedule. Is the fueler still available? Has the fueler moved on to another aircraft or even another airline so they too can get their other flights out on time? (Sometimes contract fuelers work for multiple airlines) The point is, everyone in contact with an aircraft will try their best to get their job done for a flight to depart on time as often as possible. No one wants to be the cause of a flight delay.
When an aircraft returns to the gate for something like extra fuel (when it could have been avoided), that causes potential problems for other flights which might still be running on time.
To try and avoid the aircraft returning to the gate for more fuel scenario, Flight Dispatchers may want to manually add additional taxi fuel on top of the standard taxi fuel when the flight plan is generated. (Use the “Remarks” section on the flight plan to briefly explain why you have added additional taxi fuel. You will receive less phone calls from Captains with questions.)
Taxi fuel is not part of the min fuel requirement, as it is planned to be burned before take-off. So how much additional taxi fuel should be manually added to the flight plan during deicing operations? If I were to guess, about 30 mins worth of APU burn time should be enough. So how much fuel does the APU burn in 30 mins?
The APU burn rate I came up with was based on a 45 minute observation of the fuel burn from Tank Number 1 on a Boeing B737-800 in winter. At 1517 local time, the fuel load in Tank 1 was 7000 lbs. The APU continued to run without interruptions and by 1557 local time the fuel load in Tank 1 was 6850 lbs. In a period of 40 mins, the APU burned 150 lbs. To calculate an hourly burn rate, divide 150 lbs by 40 then multiply by 60. The hourly burn rate for the B737-800 APU is approximately 225 lbs (102 kg) per hour.
As a result, I would add an additional 100 lbs (45 kg) of taxi fuel for flights that are expecting to be waiting in line for deicing. I’ve lost count how many times a Captain has come out to ask for an extra 200lbs or 300 lbs of fuel while fueling the aircraft.
The hourly burn rate for the B737-800 APU is approximately 225 lbs (102 kg) per hour.
From a fueling perspective, how much fuel on top of the requested fuel (Gate Fuel) should you add? First of all what time is it when you are fueling the aircraft and how long will this aircraft be sitting at the gate? Is the APU running or is the GPU plugged in? Is it a tankering flight? For example every flight into Chicago O’Hare (KORD) tankers as much as possible without penalizing payload or exceeding landing limits. I do not exceed tankering flights, only what they ask for, especially if it is a quick turn to KORD.
If the aircraft has arrived and the APU is left running; I use 200 lbs an hour to guesstimate how much fuel to add in additional to the requested gate fuel. If I know the aircraft is going to sit at the gate with the APU running for 45 mins to an hour, I’ll add 200 lbs in additional to the original requested fuel. If its a quick turn, to be sitting at the gate for less than 30 mins, I’ll only add 100lbs in addition to the requested fuel.
It also depends what aircraft type you are fueling too. Some aircraft such as the B737-800 and A319 can be fueled more accurately compared to the CRJ Regional Aircraft series due to the fueling system onboard the aircraft.
I have not had a chance to compare or measure APU fuel burn rates on the A319, A320, ERJ145, ERJ175, CRJ200, CRJ700 or CRJ900. I am guessing the APU fuel burn rate for the A319 and A320 is probably similar to the B737, due to the similarity in aircraft sizes. As for the ERJ and CRJ series aircraft, I don’t know if the APU burn rate is similar or not?
Overall I use 200lbs an hour to roughly calculate how much fuel to add in additional to the original gate fuel. The reason is to try and avoid being called back to add an additional 20lbs or 50 lbs of fuel. That has happened before. (If additional fuel is needed for an operational change, then that’s different and does not apply here.) When it is cold, wet and windy outside; the last thing you want to do is fuel an aircraft you have already fueled because the APU was left running during a crew change or aircraft swap. The fueling hoses can be quite heavy, especially if the fueling callback could have been avoided.