the screen is flimsy and doesn't hold up properly.

I do have it on my lap open at 90 degree angle

Then your macbook Air is 100% normal 😉 (see below)

The friction hinges on a Macbook Air are designed to be "looser than" a Macbook Pro for very sound mechanical and design reasons.

however no Mac is "meant for your Lap" as per Apple and mechanical logic of how they work.

IF the Air was designed with a very tight hinge this would cause issues with the thin nature of the fragile LCD screen, and both its opening and closing. Such flexing over time is defeating to the fragile LCD in its thinner (than the Macbook Pro) alloy housing.

get one of these for use on your lap (however that still makes it unstable re: the screen movement)

Lap desk for use with your notebook on the bed, your lap, or couch. Also very useful for removing wrist strain

Your MacBook Air is a notebook, not a laptop

When you’re using your MacBook or charging the battery, it's normal for the bottom of the case to get warm. For prolonged use, place your Macbook on a flat, stable surface. Do not place your Macbook on your lap or other body surface for extended periods of time. Prolonged body contact can cause discomfort and potentially a burn. The bottom of the Macbook case functions as a cooling surface that transfers heat from inside the computer to the cooler air outside. The bottom of the case is raised slightly to allow airflow, which keeps the unit within normal operating temperatures. In addition, warm air is vented from the slots in the back of the case. This is why your Macbook is a notebook and not a laptop, additionally the monitors on Macbook notebooks do not lay flat as in the case of many laptops. Again, use a lapdesk if you want or plan on using your Macbook on your lap.

You have it on your lap? then its normal

on a flat table, then not so

As per Apple.com

Learn what you can do if you experience one or more of these issues with the hinges on your MacBook Air.

• Unable to close lid completely.
• Broken or cracked plastic near one or both of the hinges.
• More than one inch (2.54cm) of free play while opening or closing the lid.
• Lid falls freely into closed position from a 30-degree open position. (From a closed position, open the lid approximately 30 degrees and let go.)

END from Apple.com

IF you are LIFTING your Air up and claiming that the monitor will flop open at or past 90 degrees. Normal.

or the same if youre using on your lap, couch etc

You said------falls back when open

Be very specific, Define "when open" at what angle?

Define WHAT it is sitting on.....while lifting? or while sitting?

Due to the thin nature of the LCD backlit LED monitor on the Air, you would NOT WANT a very tight hinge.

Why? Excessive hinge resistance would cause the screen to BOW (and possibly crack), especially on a longer fulcrum from the hinge in the case of the 13" screen...... Ergo it is designed logically to be “looser than” a traditional macbook Pro's hinges for a very good reason.

IF on a flat surface "just past 90 degrees" it "falls on its own" that is ABnormal

differentiate WHERE/HOW it is opening up at, on a flat surface, then its ABnormal

If you mean the friction hinges seem loose and the screen does not stay at a set angle, then take it in to Apple and have it services. That is what your warranty is for. The hinges can occasionally work loose, and they can be snugged back up (or replaced if actually defective).

~*~*~* waves back 😊