diff --git a/Makefile.ac b/Makefile.ac
index 4644b38ab0..5f3f2ec4ea 100644
--- a/Makefile.ac
+++ b/Makefile.ac
@@ -236,11 +236,8 @@ $(SETTINGS_TELEMETRY) : $(PERIODIC_H)
 %.upload: %.compile
 	cd $(AIRBORNE); $(MAKE) TARGET=$* upload
 
-jsbsim jsbsim.compile: jsbsim.ac_h print_version
-	cd $(AIRBORNE); $(MAKE) TARGET=jsbsim ARCHI=jsbsim ARCH=jsbsim all
-
-sim sim.compile: sim.ac_h print_version
-	cd $(AIRBORNE); $(MAKE) TARGET=sim ARCHI=sim ARCH=sim all
+sim sim.compile: sim.ac_h | print_version
+	cd $(AIRBORNE); $(MAKE) -j$(NPROCS) TARGET=sim ARCHI=sim ARCH=sim all
 
 # Rules for backward compatibility (old guys are used to !)
 fbw : fbw.compile