diff --git a/src/modules/vtol_att_control/tailsitter.cpp b/src/modules/vtol_att_control/tailsitter.cpp
index 52e9792eab..bae680c698 100644
--- a/src/modules/vtol_att_control/tailsitter.cpp
+++ b/src/modules/vtol_att_control/tailsitter.cpp
@@ -180,7 +180,7 @@ void Tailsitter::update_vtol_state()
 
 void Tailsitter::update_transition_state()
 {
-	float time_since_trans_start = (float)(hrt_absolute_time() - _vtol_schedule.transition_start) * 1e-6f;
+	const float time_since_trans_start = (float)(hrt_absolute_time() - _vtol_schedule.transition_start) * 1e-6f;
 
 	if (!_flag_was_in_trans_mode) {
 		_flag_was_in_trans_mode = true;
@@ -212,9 +212,12 @@ void Tailsitter::update_transition_state()
 		_q_trans_sp = _q_trans_start;
 	}
 
+	// ensure input quaternions are exactly normalized because acosf(1.00001) == NaN
+	_q_trans_sp.normalize();
+
 	// tilt angle (zero if vehicle nose points up (hover))
-	float tilt = acosf(_q_trans_sp(0) * _q_trans_sp(0) - _q_trans_sp(1) * _q_trans_sp(1) - _q_trans_sp(2) * _q_trans_sp(
-				   2) + _q_trans_sp(3) * _q_trans_sp(3));
+	const float tilt = acosf(_q_trans_sp(0) * _q_trans_sp(0) - _q_trans_sp(1) * _q_trans_sp(1) - _q_trans_sp(2) *
+				 _q_trans_sp(2) + _q_trans_sp(3) * _q_trans_sp(3));
 
 	if (_vtol_schedule.flight_mode == vtol_mode::TRANSITION_FRONT_P1) {