Fuel burn - effect on IAS and SAR

[flying67]

Hey guys,

I was doing a quiz for APLA and had the following question. It was asking, if fuel is burnt, what happens to IAS and SAR? I chose the answer it decreases the IAS and SAR remains the same but I dont entirely understand the question if someone is able to explain?

Thanks!

[Capn Bloggs]

if fuel is burnt, what happens to IAS and SAR?

​​​​​​​I dont entirely understand the question

Nor do I. What was the exact wording of the question?

Now if the question was "As" fuel is burnt, then that's different. Weight will reduce so IAS will increase (same power, lighter=faster). As for SAR, I would say it will increase as well (higher TAS for same FF=greater range for same FF), but the FMS is your friend! ​​​​​​​

[flying67]

I can't remember the exact wording unfortunately. But your explanation makes sense. Thank you for responding!

[NaFenn]

Originally Posted by flying67

Hey guys,

I was doing a quiz for APLA and had the following question. It was asking, if fuel is burnt, what happens to IAS and SAR? I chose the answer it decreases the IAS and SAR remains the same but I dont entirely understand the question if someone is able to explain?

Thanks!

Assuming the question was something along the lines of "For a constant temperature, altitude and power setting, what will happen to IAS and SAR as fuel is burnt". With a reduction in weight from the burnt fuel and a constant power setting (i.e. Same fuel burn), the TAS (and therefore IAS) will increase due to the reduction in drag with the same thrust setting. This will result in an increased SAR (Higher TAS with the same fuel burn, you will travel further for the same amount of fuel).

Having said that, for a fixed TAS, less thrust is required to maintain the same speed as weight reduces meaning a reduction in fuel flow. This means that for a constant speed, reducing weight will also increase SAR (Same speed with reduced fuel flow, you will travel further for the same amount of fuel).

The calculation for SAR for APLA is TAS/FF * 1000, meaning that as TAS increases for a fixed fuel flow, or if Fuel Flow reduces for a fixed TAS the SAR will increase meaning you will fly further for the same Fuel on Board.

Both of these assume the same temperature and altitude, which for a 1 marker (which this sounds like) is likely to be the case.

[flying67]

Originally Posted by NaFenn

So, the question as is doesn't make sense, but I will do my best here:

Assuming the question was something along the lines of "For a constant temperature, altitude and power setting, what will happen to IAS and SAR as fuel is burnt". With a reduction in weight from the burnt fuel and a constant power setting (i.e. Same fuel burn), the IAS will increase. This will also result in a reduced SAR (Higher speed with the same fuel burn, you burn less fuel per mile).

Having said that, for a fixed speed, less thrust is required to maintain the same speed as weight reduces meaning a reduction in fuel flow. This means that for a constant speed, reducing weight will also reduce SAR (Same speed with reduced fuel flow, you will burn less fuel per mile).

It seems a little bit backwards, but as SAR increases your aircraft range reduces (an SAR of 1.0 would burn 1kg/nm, where an SAR of 2.0 would burn 2kg/nm - meaning you would use twice as much fuel for the same distance, therefore reducing your range).

Both of these assume the same temperature and altitude, which for a 1 marker (which this sounds like) is likely to be the case.

Okay this does make sense - as fuel is burnt the IAS increases regardless (because of the weight reduction, plane flies faster). I think I am still a bit confused with the SAR then. Logically it makes sense that your range reduces as fuel is burnt, because less fuel = less distance flown. But in the other post it says SAR increases? Sorry for the confusion and lack of specific question

[NaFenn]

To be perfectly honest - I have found references that refer to both ways (which is confusing as all hell having not looked at the ATPL stuff for a number of years, and given the practical application is usually the inverse). But Capn Bloggs is correct, SAR increasing = Increased range (which makes alot more sense now I look back on it). Practically in flight we use the inverse as it makes more sense for ease of calculations, which is also used in AFPA - hence me having it backwards in the above post.

So, SAR is TAS/FF x 1000 which will give Nm/1000kg of fuel. Meaning that an increase in SAR = an Increase in Range, meaning For a Fixed thrust setting (and the same atmospheric conditions) as fuel is burnt, reducing weight, TAS (and therefore IAS) will increase with the reduced drag and SAR will increase as you are going further on the same amount of fuel.

I have also corrected the above post (But happy to admit error).