Approach Speed Calculation with Weapons Load

[Avio]

I know of the formula for calculating approach speed with ammo and fuel loading, but what about when also carrying stores like tanks and bombs? What are the additional knots for such loads?

 

[Rudel_chw]

Use the AOA indicator, that way you don’t need to calculate at all .. the yellow donut will automatically light up at a higher speed if you are heavier.

[Baaz]

From page 205 of the Flight Manual (found in your DCS "Doc" folder):

 

"WARNING! If the remaining fuel level is more than 1000 pounds, add 1 KIAS per each 200 pounds. If the guns are fully loaded, increase the speed by 5 KIAS."

 

I know the above does not directly answer your question, but using the above rule you should be able to calculate this out.  As for determining the weight of your onboard stores, you would just have to get the weights while doing your preflight minus what you expended.

 

For those who don't know:

 

"Formula to calculate runway approach speed depending on weight and ammo:

 

Vapproach=145+5(if there is gun ammo)+(fuel remaining -1000)/200

 

EXAMPLE. If there are full gun ammo and fuel level of 3000 pounds, increase the speed by 15 KIAS. I.е. Speed before turning to landing course has to be 180 KIAS. Glide path speed has to be 160 KIAS"

[Machalot]

1 hour ago, Baaz said:

Vapproach=145+5(if there is gun ammo)+(fuel remaining -1000)/200

I interpret the instruction to omit the fuel term if less than 1000 lb remain. Otherwise you end up reducing the calculated speed by 1 kt for every 200 lbs under 1000.

[Baaz]

1 minute ago, Machalot said:

I interpret the instruction to omit the fuel term if less than 1000 lb remain. Otherwise you end up reducing the calculated speed by 1 kt for every 200 lbs under 1000.

From page 204:

 

"The typical landing approach is shown in this scheme below. Landing weight is 11700 lb (remaining fuel 1000 lb), without gun ammo." [bold added]
Under these conditions your approach speed will be 145 knots. 

 

The formula stated above is for conditions above typical.  The -1000 pounds is to remove the weight that had already been accounted for.

 

I can't say you would be wrong for reducing the speed 1 knot per 200 pounds less than 1000.

[Machalot]

2 minutes ago, Baaz said:

From page 204:

 

"The typical landing approach is shown in this scheme below. Landing weight is 11700 lb (remaining fuel 1000 lb), without gun ammo." [bold added]
Under these conditions your approach speed will be 145 knots. 

 

The formula stated above is for conditions above typical.  The -1000 pounds is to remove the weight that had already been accounted for.

 

I can't say you would be wrong for reducing the speed 1 knot per 200 pounds less than 1000.

I don't dispute the formula is correct for 1000 lbs of fuel and above. The reason I read it the way I do is the fuel weight modification is stated as "if remaining fuel level is more than 1000 lbs", and only mentions adding speed. There's no statement about subtracting speed (at least in the quoted text; I don't have access to the manual at the moment).

[Baaz]

1 minute ago, Machalot said:

There's no statement about subtracting speed (at least in the quoted text; I don't have access to the manual at the moment).

You are correct about there being no statement for having less than 1000 pounds, that's why I said, "I can't say you would be wrong..."

What is certain is I'm no SME on the F-5.  Your guess would be just as good as mine.

[Baaz]

38 minutes ago, Baaz said:

You are correct about there being no statement for having less than 1000 pounds, that's why I said, "I can't say you would be wrong..."

What is certain is I'm no SME on the F-5.  Your guess would be just as good as mine.

After further reading about stalls on pages 211-212, having an approach speed less than 145 knots could be problematic.

As discussed earlier, a typical approach begins with an approach speed at 145 knots.  Our AOA indexer is set for a 15.8 units approach.  This gives us a final touchdown speed of 135 knots.

According to page 211, "First indication of the forthcoming stall is the onset of light-intensity buffet occurring at 16 17 units AOA with maneuver flaps set to AUTO..." and further on page 212 that "The airplane typically recovers (from Post-Stall Gyration) [bold added] as soon as airspeed reaches 130 KIAS.

Based on those two statements on pages 211 and 212, I interpret them to mean that coming in at less than 145 knots on approach could lead to a stall of the aircraft prior to touchdown.

Edited March 30, 2021 by Baaz

[Avio]

Thanks all for the helpful advice. I read somewhere before that the AOA indexer shows a few knots slower than that shown by the AOA needle indicator, and as such the AOA needle is a better judge of the correct approach speed. Is that correct?