Based on your latest posting, I now know why you think that the word “killed” in the sentence “Five people have been killed in a plane crash” isn’t a verb but a past-participle adjective. That mistaken idea actually stems from your incorrect notion that “a legitimate passive voice construction proves itself to be so if it can smoothly be changed into its active voice by eliminating the preposition ‘by’ from it.” That notion is downright false, and I will hasten to add that it proceeds from a totally wrong premise.
In English, as you must know very well, there are three kinds of verbs—transitive verbs, intransitive verbs, and linking verbs—and there’s a big difference in how transitive verbs and intransitive verbs work in a sentence. Indeed, one of the blind spots that I discovered in how English is taught over the years is that teachers don’t know and learners don’t learn this unique characteristic of intransitive verbs: When an intransitive verb is used as main verb in an active-voice sentence, that sentence couldn't possibly have a passive-voice construction using that same verb.
Consider the intransitive verb “shift” in the following active-voice sentence:
“Many Internet users shifted from dialup service to broadband.”
Now try constructing that sentence into a passive-voice form using “shift” as the verb. It just can’t be possibly done. In fact, the only way to render that sentence into passive-voice form is to relieve “shift” of its job as the verb, make it the subject of the sentence instead, and let some other verb like, say, “noted” perform the action:
“A shift from dialup service to broadband was noted among many Internet users.”
In the case of the verb “kill,” it is working as a transitive verb in the following sentence:
“The heavy snow killed the flock of birds.”
In such cases, it is perfectly possible to render the sentence into the passive-voice form—by making the object of the verb “flock of verbs” as the subject of the sentence instead:
“The flock of verbs was killed by the heavy snow.”
But then we can’t do the reverse of this process—that is, render a passive-voice sentence into the active-voice form—in the case of a sentence that uses an intransitive verb as its operative verb. Try doing that to the following sentence:
“Five people have been killed.”
There’s really no way to convert that active-voice sentence into a passive-voice sentence. This, to me, is clear and incontrovertible proof that the passive-voice form “have been killed” is in itself a self-contained grammatical form , in which the verb stays permanently intransitive by virtue of being an integral component of the passive-voice form.
Of course, when that same passive-voice sentence is modified with the addition of a prepositional phrase complement like, say, “in a plane crash,” it becomes seductive to entertain the notion—a patently wrong one—that the passive-voice sentence could have an active-voice equivalent. Indeed, the following sentence of yours,
“Five people have been killed in a plane crash”
can be correctly rendered in the following active-voice form:
“A plane crash has killed five people.”
But then a journalistic sleigh of hand—some would call it changing the grammatical goalpost—has been committed in that active-voice rendition. The event itself —“a plane crash”—has been made the doer of the action, something that semantically speaking is far from the intended sense and meaning of your original passive-voice sentence, “Five people have been killed in a plane crash.”
The situation would be different, of course, if your original sentence were in the following form, where “a plane crash” is the doer of the action or the agent that did the killing:
“Five people have been killed by a plane crash.”
This time we can legitimately render that passive-voice sentence in the active-voice form, with “a plane crash” becoming both the subject and doer of the action, and “five people” becoming the direct object of the verb “killed”:
“A plane crash has killed five people.”
But we should note clearly here that “kill” is not anymore working as an intransitive verb but as a transitive verb, which as we know always requires a direct object. (As we know, many verbs in English have that dual character depending on how they are used in a sentence.) What has happened is that by the simple expedient of using the preposition “by” instead of “in” in the passive-voice sentence, we changed “a plane crash” from object of the preposition to doer of the action. In the process, “kill” got transformed from an intransitive verb into a transitive verb, thereby fundamentally changing the grammatical ballgame in that sentence, so to speak.
This, I must add, is the danger of unilaterally breaking up sentences and their syntactical elements for grammatical analysis, as you have done at the end of your latest posting. The very act of breaking them up very often changes the character of the component elements, in much the same way that electrolysis breaks up water into hydrogen and oxygen, which of course are entirely different chemical elements with entirely different behaviors.
