empty weight, and a freedom-giving
448 less than the MTOW. Once, having
taken off in a loose formation with
another fully loaded Ercoupe at St. Paul
Downtown Airport (STP) in Minnesota,
he had the pleasure of finding himself
500 feet above his companion as they
crossed the far threshold of the runway.
FORMULA? WHAT FORMULA?
In the early days of ultralighting, most
machines were operated pretty close to
maximum gross weight most of the
time. Official MTOW numbers were
not always available, and factory-sup-plied numbers were not always
reliable, if given. For many designs,
such things as MTOWs, speeds, fuel
consumption, and takeoff and landing
distances were targets rather than
something actually measured.
Some manufacturers tested their
designs to destruction, if they could
afford to destroy one. To find ultimate
breaking strength, an aircraft would
typically be suspended upside down
and sandbags piled on the wings until
something broke. Some sort of formula
would be applied to the weight at
which it broke to determine how much
of a margin to leave for the pilot and
fuel. What formula, you ask? That was
up to the manufacturer, since these
were not federally regulated vehicles.
In the type-certificated world, the
MTOW would normally be 66. 6 percent
of the weight that it took to produce
structural failure. In the ultralight and
homebuilt world, it might have been 75
percent, or 80 percent, or 60 percent.
Not all ultralight manufacturers spelled
out how they derived their numbers. To
be fair, some manufacturers, such as
Quicksilver, followed generally
accepted airworthiness standards. The
situation is far better today
industrywide, with LSA meeting a
specific set of standards developed by
the ASTM LSA committees.
On the performance side of MTOW,
almost any aircraft can be over-gross
and still struggle into the air, but that
doesn’t mean anyone should attempt it.
Cool, dry weather, a head wind, and a
long, smooth runway will allow a
heavier-than-gross airplane to get
airborne. And being heavy—whether
over-gross or not—will lengthen landing
distances. However, in practice,
increased takeoff and landing distances
are not likely to affect lightweights very
much when they operate out of airfields
made for heavier general aviation
aircraft, which are ordinarily more than
long enough.
It’s the part between the takeoff and
the landing that’s of great concern
because, once airborne, the airframe is
at risk of structural failure.
While having a specific number for
MTOW is good, we are probably better
off regarding higher weights as a range,
rather than fixating on one number,
making it a sharp dividing line and
saying, “Less than this is okay, more is
not.” For example, if the maximum
allowable takeoff weight for your
airplane is 850 pounds, any actual weight
that even approaches 850 should be
regarded with skepticism. The 850 was
based on the assumption that the
airplane still has all of the strength
originally designed into it. Some of that
strength may have been lost over the
It’s the part in between the
takeoff and the landing that’s
of great concern because, once
airborne, the airframe is at risk
of structural failure.
years to wear and tear, possibly some
corrosion, and the occasional flight into
turbulence that might have stretched and
torqued the airframe.
Speaking of turbulence, one more
point: If you fly your 850-pound
airplane into a sudden upward gust, it is
going to “weigh” a lot more than 850 for
the duration of the gust. That is a
sobering thought.
