engine operation under negative load factor
Dear Forum readers
While reading the book "Aircraft Structures by Peery and Azar". On the chapter about flight loads there is a sentence which I don't fully understand
"The negative high angle of attack condition, occurs in intentional flight maneuvres in which the air loads on the wing are down or when the airplane strikes sudden downdrafts while in level flight. The load factors for intentional negative flight atitudes are considerably smaller than for positive flight attitudes, because conventional aircraft engines cannot be operated under a negative load factor for very long..."
Although this is stated as an evidence, what's the main reason why an engine cannot be operated under negative load factor for long?
Thanks for help
Regards