  Aql X--1 is one of the most prolific low mass X-ray binary transients
(LMXBTs) showing outbursts almost annually. We present the results of our
spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts.
We investigate the spectral changes related to the changing disk-magnetosphere
interaction modes of Aql X--1. The X-ray light curves of the outbursts of
LMXBTs typically show phases of fast rise and exponential decay. The decay
phase shows a "knee" where the flux goes from the slow decay to the rapid decay
stage. We assume that the rapid decay corresponds to a weak propeller stage at
which a fraction of the inflowing matter in the disk accretes onto the star. We
introduce a novel method for inferring, from the light curve, the fraction of
the inflowing matter in the disk that accretes onto the NS depending on the
fastness parameter. We determine the fastness parameter range within which the
transition from the accretion to the partial propeller stage is realized. This
fastness parameter range is a measure of the scale-height of the disk in units
of the inner disk radius. We applied the method to a sample of outbursts of Aql
X--1 with different maximum flux and duration times. We show that different
outbursts with different maximum luminosity and duration follow a similar path
in the parameter space of accreted/inflowing mass flux fraction versus fastness
parameter.
