Engineer reported safety concerns with the Gulfstream GVII-G500 and Gulfstream GVII-G600 Airplane Flight Manual related to the Accelerate-Go and Accelerate Stop distances.

Narrative

We identified an unsafe and potentially catastrophic condition in the Gulfstream GVII-G500 and Gulfstream GVII-G600 Airplane Flight Manual.We contacted the Original Equipment Manufacturer Gulfstream to inform them of the unsafe condition; and did not receive qualitative justification; leading to believe the unsafe condition is still present. Gulfstream promotes that the 'Appendix A1: Reduced Thrust (FLEX) Takeoff'; located in the GVII-500 and GVII-G600 Airplane Flight Manual; can be used as an alternative to the Chapter 5 - Performance; to determine allowable takeoff weight from short aircraft. To that effect; Gulfstream has also convinced airport performance providers such as Airport Performance Group (APG) to use this supplement as an alternative to Chapter 5. Customers use those blindly.This could have devastating consequences as a quick examination of the Accelerate-Go and Accelerate-Stop distances in the Chapter 5 will clearly indicate that this method does not appropriately respect necessary basic conditions to allow a safe takeoff. Example: 5;000 ft. runway at 50 ft. elevation; WET runway and 15 degrees Celsius. The Appendix A1 tells the operator the following: At the next higher pressure altitude in the chart (1000 ft because interpolation is not accepted). Takeoff flaps 20. Takeoff weight: 60;000 lb. FLD LENGTH (FT) 4970 (although not clear or explained what this parameter incorporates). V1 KIAS 116. VR KIAS 118. V2 KIAS 133.However; when reporting those parameters in the Chapter 5; at this specific V1 over VR ratio (0.983); an effective runway length of 5;000 ft.; we get that the GVII-G500 aircraft would need 4;200 ft. of runway for the Accelerate-go; but would require 6;400 ft. for the Accelerate-stop distance; putting public at great risk if the pilot had to make the decision to stop the aircraft right before V1. This situation is of great concern. The FLD LENGTH parameter in the Appendix A1 does not consider the higher value of the Accelerate-go or accelerate-stop and is therefore NOT conservative. Furthermore; the said supplement does not offer an example on how a pilot would use such a supplement in lieu of using Chapter 5 to calculate performance. All of which poses a great danger to public safety and is an accident waiting to happen.