David Ray Griffin isn’t convinced by the claim that NORAD didn’t recognise the threat of a hijacked US airliner being used as a weapon:

The contention that the threat really was not recognised was based partly on the fact that the Kean-Zelikow Report combined the second and third presumption into a single twofold presumption--that commercial airliners would not be (1) hijacked within the United States and then (2) used as guided missiles by suicide hijackers. Given the fact that both conditions would need to be met to have a counter-instance to Eberhart’s “no recognition” claim, the Kean-Zelikow Report can dismiss seeming counter-instances by pointing out that one or the other of the two conditions was not met. The Report mentioned, for example, a proposed readiness test for NORAD based on the idea of “a hijacked airliner coming from overseas and crashing into the Pentagon” (346). This example, by having the aircraft come from overseas, provided no refutation of the contention that no one had imagined a plane hijacked within the United States and then used to strike the Pentagon--the crucial difference being that if the plane were coming from overseas, there would be plenty of time to identify the aircraft and scramble interceptors.

However, even with the stipulation that both conditions would have to be met, there is considerable evidence that counts against the credibility of Eberhart’s claim. 

Some of this evidence is, surprisingly enough, provided by the Commission itself. I will list nine examples provided in the Kean-Zelikow Report that either clearly do, or at least may, contradict the Report’s endorsement of Eberhart’s “no recognition” claim.

Page 263
The 9/11 Commission Report: Omissions and Distortions
David Ray Griffin

One interesting point here is his mention of the “proposed readiness test for NORAD based on the idea of ‘a hijacked airliner coming from overseas and crashing into the Pentagon’ (346)”. The way Dr Griffin tells it, this would appear to be the point of the exercise, however, as we’ve seen, it was a side issue that never actually happened:

One idea, intended to test command and control plans and NORAD's readiness, postulated a hijacked airliner coming from overseas and crashing into the Pentagon.The idea was put aside in the early planning of the exercise as too much of a distraction from the main focus (war in Korea), and as too unrealistic.
Page 346
9/11 Commission Report

He also appears to be implying that simply providing examples of the possible use of planes hijacked in the US, then being used as weapons, will contradict Eberhart’s “no recognition” claim. But is that true? 

Here’s where the claim is used in the Commission Report:

NORAD perceived the dominant threat to be from cruise missiles. Other threats were identified during the late 1990s, including terrorists' use of aircraft as weapons. Exercises were conducted to counter this threat, but they were not based on actual intelligence. In most instances, the main concern was the use of such aircraft to deliver weapons of mass destruction.

Prior to 9/11, it was understood that an order to shoot down a commercial aircraft would have to be issued by the National Command Authority (a phrase used to describe the president and secretary of defense). Exercise planners also assumed that the aircraft would originate from outside the United States, allowing time to identify the target and scramble interceptors. The threat of terrorists hijacking commercial airliners within the United States -- and using them as guided missiles -- was not recognized by NORAD before 9/11.98
Page 17
9/11 Commission Report

They’re plainly conceding that there was knowledge of the potential threat from suicide hijacks. The Commission tell us that NORAD exercise planners assumed that hijacked aircraft would originate from outside the US, but they don’t say that no-one else ever suggested such a thing. 

This is what General Myers said at the 9/11 Commission hearings:

MR. LEHMAN: But when you were NORAD commander, there had already been a private aircraft that crashed into the White House grounds. There were repeated and written worries about the potential for private aircraft to make suicide attacks, and there were 11 separate intelligence reports circulating broadly through the intelligence community that al Qaeda had planned to use aircraft as weapons, although the focus was overseas. Didn't anybody at NORAD try to connect the dots and say that this is something we've got to worry about, that it's a target in the Capitol area, that we'd better get ready for it? But, instead, when even NORAD's own planning staff proposed to include in exercises the dealing with hijacked suicide aircraft, it was rejected by NORAD as by the NORAD commander, I think it was after your time, as something to be exercised and planned for.

GEN. MYERS: I think it was rejected, and General Eberhart can be clearer on this, I don't think it was by the commander, I think it was by the planning group that was meeting because it did not fit the scenario at the time. But, the use of aircraft as a weapon, as a missile, other than World War II and the Kamikaze situation, I'm not aware, and I've tried to research this, and the best information I get, I am not aware that an aircraft has ever been used as a weapon. Now, there have been landings on the White House lawn, there was a landing in Red Square, there have been lots of stupid things. There was talk about crashing airplanes into the CIA. But, in most of that threat reporting leading up to 9/11, it was hijacking an airplane and in the normal hijack mode, not in the mode of a weapon.

Now, there were some talks about in post hijack situations where they talked to about people over the demands were made that they were going to crash, one instance, into the Eiffel Tower, but even the work that was done and the hijackings that were planned for the Philippines, which is a well-known plot, they planned to hijack the airplanes and blow them up primarily.

So, no, the threat perception, there was not -- the intelligence did not point to this kind of threat, and I think that explains our posture.
http://www.9-11commission.gov/archive/hearing12/9-11Commission_Hearing_2004-06-17.htm

Myers is also conceding that there were reports about using planes as weapons, but the intelligence didn’t indicate these were likely.

To consider the Commission sentence again, then:

The threat of terrorists hijacking commercial airliners within the United States -- and using them as guided missiles -- was not recognized by NORAD before 9/11

We would say “recognised” here is used in the sense of “accepted as true or valid” (see definitions here). And as such, the Commission weren’t saying NORAD were unaware of any suggestions that terrorists might hijack commercial airliners and use them as weapons, or that no-one had imagined such a possibility, just that they didn’t accept this as a credible threat.

If we’ve interpreted this correctly, then, to prove the Commission wrong will take far more than simply pointing to someone suggesting the possibility of suicide hijackings in the US. But then Dr Griffin has told us he’s got nine examples from the 9/11 Commission Report alone, so perhaps we should start by reviewing those.